2. 2
Mechanical Drives/Power Transmission Training
The purpose of this presentation is to
provide a overview of industrial drives and
their maintenance.
Gear Drives
Belt Drives
V-Belt Maintenance
Chain Drives
Sprocket and Roller Chain Maintenance
7. 7
Mechanical Drives/Power Transmission Training
Gears and Gear Boxes
A gear is a form of disc, or wheel, that has
teeth around its periphery for the purpose
of providing a positive drive by meshing
the teeth with similar teeth on another
gear or rack.
11. 11
Mechanical Drives/Power Transmission Training
Pressure Angle
Pressure
Angle
Pressure
angle
Pressure
angle
Rotation
Line of
action
14 ½ ° 20°
Direction of
tooth to
tooth push
12. 12
Mechanical Drives/Power Transmission Training
Pitch Diameter and Center
Distance
2
21 DD
C
+
=
221 DCD −=
Center Distance C
Pitch
Diameter D1
Pitch
Diameter D2
122 DCD −=
14. 14
Mechanical Drives/Power Transmission Training
Names of Gear Parts
Clearance
Pitch
Circle
Addendum
Pitch
Circle
Dedendum
Whole
depth
Working
depth
Thickness
Circular
pitch
Pitch
Line
Addendum
Dedendum Whole
depth
Thickness
Circular
pitch
27. 27
Mechanical Drives/Power Transmission Training
Belt Drives
Belt drives are used to transmit power
between a drive unit and a driven unit
Drive Motor
Driven Unit
28. 28
Mechanical Drives/Power Transmission Training
Belts
The size of the belt must match the
sheave size. if they do not match, then the
belt will not make proper contact with the
sheave and will decrease the amount of
load it can transmit.
A
AB
B
37. 37
Mechanical Drives/Power Transmission Training
Belt Maintenance
Never force belt onto sheave
Belt should not ride on bottom of sheave.
Check belt condition
Belt alignment
Tension
Inspect sheaves and shafts
Belt stretch
60. 60
Mechanical Drives/Power Transmission Training
Advantages
No slippage
More compact than belt
Good for slow speeds
Easier to install
Not subject to deterioration
Operates under wet conditions
Less adjustment required
Can drive several shafts
Less costly than gear drives
61. 61
Mechanical Drives/Power Transmission Training
Disadvantages
Cannot be used where drive must slip
Cannot except misalignment
Noisy
Causes vibration
Needs frequent lubrication
Small load capacity
Shorter Service Live
81. 81
Mechanical Drives/Power Transmission Training
Chain Length
The following information is needed for an
equation to find the chain length:
1. Number of teeth on the drive sprocket.
2. Number of teeth on the driven sprocket.
3. Center-to-center distance between the shafts.
4. The chain pitch in inches.
The purpose of this presentation is to provide an overview of the general types of drives used industrially.
Page <number>
Basic Rotating Equipment Training
These drives are used to provide a variable output speed from a constant-speed power source (as in a machine tool driven by a constant-speed ac motor) or to provide torque increase for a variable-speed power source (as in an automobile). Variable mechanical drives are less costly than competing electrical variable-speed drives and their control is much simpler. But mechanical drives often are not as durable and cannot be controlled as precisely as electrical drives. And except for gear drives, the mechanical type generally cannot transmit as much power as electrical drives when variable speed is essential.
The basic types of adjustable-speed drives are geared transmissions (which provide only specific fixed-speed ratios), variable-pitch belt and chain drives (which provide infinitely variable speeds), traction drives (also infinitely variable), and fluid drives.
These are the most durable, rugged, and efficient of all adjustable-speed drives. But they are capable of providing only a specific number of fixed gear ratios. Normally chosen for applications involving heavy loads or requiring long, trouble-free life. Generally, more expensive than belt or chain drives. Gear drives are commonly classified according to end use:
Automotive transmissions: Used as main transmissions in cars, trucks, farm machinery, and earth-moving equipment. Usually provide from four to 10 speeds.
Auxiliary transmissions: Usually installed behind the main transmission to increase available ratios.
Transfer cases: Provide additional power outlets (as in a four-wheel-drive vehicle) or provide offset from normal drivelines.
Power takeoffs: Usually mounted beside main transmission and driven by an additional gear in that transmission. Similar to a transfer case.
Marine gears: Transmissions carrying power to the propeller on a marine drive. Differ from other transmissions in that they generally provide single forward and reverse speeds and use friction-type shifting clutches.
Hydraulic drives: Gearboxes connecting power source and hydraulic pumps in hydrostatic drives.
Industrial transmissions: A broad category applying to any transmission powering machinery other than that described above. Many have integral power packages, such as electric or hydraulic motors, or they may be an integral part of driven components.
Differentials: A set of gears with three independent, rotating members with a speed and torque. This definition creates two application types. The first consists of one input and two outputs. The automobile differential is the best example here. The second application type has two inputs and one output. Less well known than the first, this technique solves industrial problems when the superimposition of one motion relative to another is required, such as timing of cutoff, registration control on printing presses, tension control, and phase shifting on textile industry equipment.
A mechanical fixed-speed drive still offers the best combination of rugged and reliable speed reduction at low cost. However, one factor, such as lubrication, speed, or durability should set the speed reducer apart as the clear choice for an application.
A gearbox may have one or more gear pairs. The gear pairs may be on parallel or nonparallel axes and on intersecting or nonintersecting shafts. If it has more than two pairs, the setup is called a gear train. Generally, they permit higher speed ratios in smaller packages than are feasible with a single pair of gears.
Series trains: Overall ratio is input shaft speed divided by output speed. It is also the product of individual ratios at each mesh, except in planetary gears. Ratio is most easily found by dividing the product of numbers of teeth of driven gears by the product of numbers of teeth of driving gears.
Planetary gearing: Also called epicyclic gearing. It is a gear train in which a planet gear rotates on its own axis while its axis rotates about another gear called the sun gear. The name epicyclic gearing is derived from the epicycloid curve which is generated by a point on the surface of the planet gear as it rotates about the sun gear. Generally, the more planet gears, the greater the torque capacity of the system.
Speed reducers:
Gearboxes usually offer only a single, fixed-reduction ratio. When they have more than one ratio, they differ from transmissions in that the reducer usually is not shifted as often or as easily. As the name implies, a reducer is almost always used to gear "down."
Speed reducers come in two varieties: base mounted and shaft mounted. The shaft-mounted type, in turn, has two variations. One is truly shaft mounted in that it is supported entirely by the input shaft of the drive machine, with torque reaction absorbed by a special link. The other is mounted to the driven-machine housing so the input shaft does not absorb reducer weight or torque reaction. By AGMA definition, the term "speed reducer" is applied to units operating at pinion speeds below 3,600 rpm or pitch-line velocities below 5,000 fpm. Reducers operating at speeds higher than these are called "high-speed units."
Speed increasers: These gearboxes require special care in design and manufacturing. They often involve high speeds which may create problems in gear dynamics. Also, frictional and drag forces are magnified.
This is an example of a Falk type DTC right angle conveyor drive.
Ask the class what type of gears are used in it.
The answer is helical and spiral bevel.
After covering gear types in the following slides return here and point them out.
The spur gear might be called the basic gear, since all other types have been developed from it. Its teeth are straight and parallel to the center bore line, as shown above.
Spur gears may run together with other spur gears or parallel shafts, with internal gears on parallel shafts, and with a rack. A rack such as the one illustrated. is in effect a straight-line gear.
The smallest of a pair of gears is often called a pinion.
The involute profile or form is the one most commonly used for gear teeth. It is a curve that is traced by a point on the end of a taut line unwinding from a circle. The larger the circle, the straighter the curvature; and for a rack, which is essentially a section of an infinitely large gear, the form is straight or flat. The generation of an involute curve is illustrated.
The involute system of spur gearing is based on a rack having straight or flat sides. All gears made to run correctly with this rack will run with each other.
The shafts are in the same plane and parallel. The teeth are cut straight and parallel to the axis of the shaft rotation. No more than two sets of teeth are in mesh at one time, so the load is transferred from one tooth to the next tooth rapidly. Usually spur gears are used for moderate- to low-speed applications. Rotation of spur gear sets is opposite unless one or more idler gears are included in the gearbox. Typically, spur gear sets will generate a radial load (preload) opposite the mesh on their shaft support bearings and little or no axial load.
We use the word involute because the contour of gear teeth curves inward and is used in gear tooth profiles.
The involute curve is a curve that is traced by a point on the end of a taut line unwinding from a circle. The larger the circle, the straighter the curvature; and for a rack, which is essentially a section of an infinitely large gear, the form is straight or flat. The generation of an involute curve is illustrated.
Attach a string to a point on a curve. Extend the string so that it is tangent to the curve at the point of attachment. Then wind the string up, keeping it always taut. The locus of points traced out by the end of the string is called the involute of the original curve, and the original curve is called the evolute of its involute. This process is illustrated above for a circle.
The sides of each tooth incline toward the center top at an angle called the pressure angle as shown
The 14.5-degree pressure angle was standard for many years. In recent years, however, the use of the 20-degree pressure angle has been growing, and today 14.5-degree gearing is generally limited to replacement work.
The principal reasons are that a 20-degree pressure angle results in a gear tooth with greater strength and wear resistance and permits the use of pinions with fewer teeth. The effect of the pressure angle on the tooth
It is extremely important that the pressure angle be known when gears are mated, as all gears that run together must have the same pressure angle.
The pressure angle of a gear is the angle between the line of action and the line tangent to the pitch circles of mating gears.
The slide illustrates the relationship of the pressure angle to the line of action and the line tangent to the pitch circles.
Pitch circles have been defined as the imaginary circles that are in contact when two standard gears are in correct mesh.
The diameters of these circles are the pitch diameters of the gears. The center distance of the two gears, therefore, when correctly meshed, is equal to one half of the sum of the two pitch diameters, as shown.
This relationship may also be stated in an equation, and may be simplified by using letters to indicate the various values, as illustrated.
Have class try some example calculations.
Circular Pitch
A specific type of pitch designates the size and proportion of gear teeth. In gearing terms, there are two specific types of pitch: circular pitch and diametrical pitch. Circular pitch is simply the distance from a point on one tooth to a corresponding point on the next tooth, measured along the pitch line or circle, as illustrated. Large-diameter gears are frequently made to circular pitch dimensions.
Diametrical Pitch and Measurement
The diametrical pitch system is the most widely used, as practically all common-size gears are made to diametrical pitch dimensions.
It designates the size and proportions of gear teeth by specifying the number of teeth in the gear for each inch of the gear’s pitch diameter. For each inch of pitch diameter, there are pi (π) inches, or 3.1416 inches, of pitch-circle circumference.
The diametric pitch number also designates the number of teeth for each 3.1416 inches of pitch-circle circumference. Stated in another way, the diametrical pitch number specifies the number of teeth in 3.1416 inches along the pitch line of a gear.
Because the pitch line of a rack is a straight line, a measurement can be easily made along it. In the illustration, it is clearly shown that there are 10 teeth in 3.1416 inches; therefore the rack illustrated is a 10 diametrical pitch rack.
In many cases, particularly on machine repair work, it may he desirable for the mechanic to determine the diametrical pitch of a gear. This may be done very easily without the use of precision measuring tools, templates, or gauges. Measurements need not be exact because diametrical pitch numbers are usually whole numbers. Therefore, if an approximate calculation results in a value close to a whole number, that whole number is the diametrical pitch number of the gear.
Method 1
Count the number of teeth in the gear, add 2 to this number, and divide by the outside diameter of the gear. Scale measurement of the gear to the closest fractional size is adequate accuracy.
Have the class work out the following example. Example a gear with 56 teeth and an outside measurement of 5 13/16 inches. Answer: Adding 2 to 56 gives 58; dividing 58 by 5 gives an answer of 9 31/32 . Since this is approximately 10, it can be safely stated that the gear is a 10 diametrical pitch gear.
The list that follows offers just a few names of the various parts given to gears. These parts are shown in the illustrations.
• Addendum: Distance the tooth projects above, or outside, the pitch line or circle
• Dedendum: Depth of a tooth space below, or inside, the pitch line or circle.
• Clearance: Amount by which the dedendum of a gear tooth exceeds the addendum of a matching gear tooth.
• Whole Depth: The total height of a tooth or the total depth of a tooth space.
• Working Depth: The depth of tooth engagement of two matching gears. It is the sum of their addendums.
• Tooth Thickness: The distance along the pitch line or circle from one side of a gear tooth to the other.
Backlash
Backlash in gears is the play between teeth that prevents binding.
In terms of tooth dimensions, it is the amount by which the width of tooth spaces exceeds the thickness of the mating gear teeth.
Backlash may also be described as the distance, measured along the pitch line, that a gear will move when engaged with another gear that is fixed or immovable, as illustrated.
Normally there must be some backlash present in gear drives to provide running clearance. This is necessary, as binding of mating gears can result in heat generation. noise, abnormal wear, possible overload, and/or failure of the drive.
A small amount of backlash is also desirable because of the dimensional variations involved in practical manufacturing tolerances.
Backlash is built into standard gears during manufacture by cutting the gear teeth thinner than normal by an amount equal to one-half the required figure. When two gears made in this manner are run together, at standard center distance, their allowances combine, provided the full amount of backlash is required.
On non-reversing drives or drives with continuous load in one direction, the increase in backlash that results from tooth wear does not adversely affect operation. However, on reversing drive and drives where timing is critical, excessive backlash usually cannot be tolerated.
Many styles and designs of gears have been developed from the spur gear.
While they are all commonly used in industry, many are complex in design and manufacture. Only a general description and explanation of principles will be given, as the field of specialized gearing is beyond the scope of this book.
Commonly used styles will be discussed sufficiently to provide the millwright or mechanic with the basic information necessary to perform installation and maintenance work.
Two major differences between bevel gears and spur gears are their shape and the relation of the shafts on which they are mounted.
The shape of a spur gear is essentially a cylinder, while the shape of a bevel gear is a cone.
Spur gears are used to transmit motion between parallel shafts, while bevel gears transmit motion between angular or intersecting shafts. The diagram in Figure 14.19 illustrates the bevel gear’s basic cone shape. Figure 14.20 shows a typical pair of bevel gears.
Special bevel gears can be manufactured to operate at any desired shaft angle.
Miter gears are bevel gears with the same number of teeth in both gears operating on shafts at right angles or at 90 degrees.
The diametrical pitch number, as with spur gears, establishes the tooth size of bevel gears.
Because the tooth size varies along its length, it must be measured at a given point. This point is the outside part of the gear where the tooth is the largest.
Because each gear in a set of bevel gears must have the same angles and tooth lengths, as well as the same diametrical pitch, they are manufactured and distributed only in mating pairs.
Bevel gears, like spur gears, are manufactured in both the 14.5-degree and 20-degree pressure-angle designs.
Helical gears are designed for parallel-shaft operation like the pair shown. They are similar to spur gears except that the teeth arc cut at an angle to the centerline.
The principal advantage of this design is the quiet, smooth action that results from the sliding contact of the meshing teeth. A disadvantage, however, is the higher friction and wear that accompanies this sliding action.
The angle at which the gear teeth are cut is called the helix angle and is illustrated.
The shafts are in the same plane and parallel but the teeth are cut at an angle to the centerline of the shafts. Helical teeth have an increased length of contact, run more quietly and have a greater strength and capacity than spur gears. Normally the angle created by a line through the center of the tooth and a line parallel to the shaft axis is 45 degrees. However, other angles may be found in machinery. Helical gears also have a preload by design; the critical force to be considered, however, is the thrust load (axial) generated in normal operation
The worm and worm gear, are used to transmit motion and power when a high-ratio speed reduction is required.
They provide a steady quiet transmission of power between shafts at right angles.
The worm is always the driver and the worm gear the driven member.
Like helical gears, worms and worm gears have “hand.”
The hand is determined by the direction of the angle of the teeth.
Thus, in order for a worm and worm gear to mesh correctly, they must be the same hand.
To overcome the disadvantage of the high end thrust present in helical gears, the herringbone gear was developed.
It consists simply of two sets of gear teeth, one right-hand and one left-hand, on the same gear.
The gear teeth of both hands cause the thrust of one set to cancel out the thrust of the other.
Thus, the advantage of helical gears is obtained, and quiet, smooth operation at higher speeds is possible.
Obviously they can only be used for transmitting power between parallel shafts.
Commonly called “double helical” because they have teeth cut with right and left helix angles, they are used for heavy loads at medium to high speeds. They do not have the inherent thrust forces that are present in helical gear sets. Herringbone gears, by design, cancel the axial loads associated with a single helical gear. The typical loads associated with herringbone gear sets are the radial side-load created by gear mesh pressure and a tangential force in the direction of rotation.
Many machine trains utilize gear drive assemblies to connect the driver to the primary machine. Gears and gearboxes typically have several vibration spectra associated with normal operation. Characterization of a gearbox’s vibration signature box is difficult to acquire but is an invaluable tool for diagnosing machine-train problems.
All gear sets create a frequency component, called gear mesh.
The fundamental gear mesh frequency is equal to the number of gear teeth times the running speed of the shaft.
In addition, all gear sets will create a series of sidebands or modulations that will be visible on both sides of the primary gear mesh frequency.
In a normal gear set, each of the sidebands will be spaced at exactly thel1X or running speed of the shaft and the profile of the entire gear mesh will be symmetrical.
Normal Profile
The sidebands will always occur in pairs, one below and one above the gear mesh frequency. The amplitude of each of these pairs will be identical.
If the gear mesh profile were split by drawing a vertical line through the actual mesh, i.e., the number of teeth times the input shaft speed, the two halves would be exactly identical. Any deviation from a symmetrical gear mesh profile is indicative of a gear problem. However, care must be exercised to ensure that the problem is internal to the gears and induced by outside influences. External misalignment, abnormal induced loads and a variety of other outside influences will destroy the symmetry of the gear mesh profile. For example, the single reduction gearbox used to transmit power to the mold oscillator system on a continuous caster drives two eccentrics. The eccentric rotation of these two cams is transmitted directly into the gearbox and will create the appearance of eccentric meshing of the gears. The spacing and amplitude of the gear mesh profile will be destroyed by this abnormal induced load.
One of the primary causes of gear failure is the fact that, with few exceptions, gear sets are designed for operation in one direction only. Failure is often caused by inappropriate bidirectional operation of the gearbox or backward installation of the gear set. Unless specifically manufactured for bidirectional operation, the “non-power” side of the gear’s teeth is not finished. Therefore, this side is rougher and does not provide the same tolerance as the finished “power” side.
Gear overload is another leading cause of failure. In some instances, the overload is constant, which is an indication that the gearbox is not suitable for the application. In other cases, the overload is intermittent and only occurs when the speed changes or when specific production demands cause a momentary spike in the torsional load requirement of the gearbox.
Misalignment, both real and induced, is also a primary root cause of gear failure. The only way to assure that gears are properly aligned is to “hard blue” the gears immediately following installation. After the gears have run for a short time, their wear pattern should be visually inspected. If the pattern does not conform to vendor’s specifications, alignment should be adjusted.
Poor maintenance practices are the primary source of real misalignment problems. Proper alignment of gear sets, especially large ones, is not an easy task. Gearbox manufacturers do not provide an easy, positive means to assure that shafts are parallel and that the proper center-to-center distance is maintained.
Induced misalignment is also a common problem with gear drives. Most gearboxes are used to drive other system components, such as bridle or process rolls. If misalignment is present in the driven members (either real or process induced), it also will directly affect the gears. The change in load zone caused by the misaligned driven component will induce misalignment in the gear set. The effect is identical to real misalignment within the gearbox or between the gearbox and mated (i.e., driver and driven) components.
Visual inspection of gears provides a positive means to isolate the potential root cause of gear damage or failures. The wear pattern or deformation of gear teeth provides clues as to the most likely forcing function or cause. The following sections discuss the clues that can be obtained from visual inspection.
The first picture illustrates a gear that has a normal wear pattern. Note that the entire surface of each tooth is uniformly smooth above and below the pitch line.
Abrasion creates unique wear patterns on the teeth. The pattern varies, depending on the type of abrasion and its specific forcing function. The second picture illustrates severe abrasive wear caused by particulates in the lubricating oil. Note the score marks that run from the root to the tip of the gear teeth.
Water and other foreign substances in the lubricating oil supply also cause gear degradation and premature failure. The first picture illustrates a typical wear pattern on gears caused by this failure mode.
The wear patterns generated by excessive gear loading vary; but all share similar components. The second picture illustrates pitting caused by excessive torsional loading. The pits are created by the implosion of lubricating oil. Other wear patterns, such as spalling and burning, can also help to identify specific forcing functions or root causes of gear failure.
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Basic Rotating Equipment Training
V-belts are utilized to transfer energy from a driver to the driven and usually transfer one speed ratio to another through the use of different sheave sizes.
V-belts are constructed for three basic components, which vary from maker to maker:
Load carrying section to transfer power.
Rubber compression section to expand sideways in the groove.
Cover of cotton or synthetic fiber to resist abrasion.
Understanding the construction of V-belts assists in the understanding of belt maintenance.
The standard V-belt must ride in the sheave properly. If the belt is worn or the sheave is worn, then you will have slippage of the belt and transfer of power, and speed will change resulting in a speed change to a piece of equipment.
If a V-belt drive is located near oil, grease, or chemicals the V-belts could lose their capability through the deterioration of the belt material, again resulting in the reduction of energy transfer and quickly resulting in belt breakage or massive belt slippage.
Belt drives are used to transmit power between a drive unit and a driven unit. For example, if we have an electric motor and a contact roll on a conveyor, we need a way to transmit the power from the electric motor to the roll. This can be done easily and efficiently with a belt drive unit.
Belt drives can consist of one or multiple belts, depending on the load that the unit must transmit.
The belts need to be the matched with the sheave type, and they must be tight enough to prevent slippage. Examples of the different belt and sheave sizes are as follows:
Fractional horsepower V-belts: 2L, 3L, 4L, and 5L;
Conventional V-belts: A, B, A-B, C, D, and E; Conventional cogged V-belts: AX, BX, and CX;
Narrow V-belts: 3V, 5V, and 8V; Narrow cogged V-belts: 3VX and 5VX;
Power band belts: these use the same top width designations as the above belts, but the number of hands is designated by the number preceding the top width designation. For example, a 3-ribbed 5V belt would be labeled 3/5V;
Positive-drive belts: XL, L, H, XH, and XXH.
The size of the belt must match the sheave size. if they do not match, then the belt will not make proper contact with the sheave and will decrease the amount of load it can transmit.
Belts are best suited for transmitting light loads between short range sheaves. They are excellent at absorbing shock. When an overload occurs, they will act as an overload device and slip, thereby protecting valuable equipment. They are also much quieter than other power transmission devices such as chains.
Because of their design, they are easier to install and maintain than other belt types. Other than an occasional re-tensioning, V-belts are virtually maintenance free. When properly installed and maintained, V-belts will provide years of trouble-free operation.
Cogged belts provide even longer life than conventional V-belts. Because of their design, they run cooler than conventional belts, thereby increasing the overall life of the belt.
Joined or power band belts provide a good alternative in pulsating drives where standard V-belts have a tendency to turn over. They function like a standard V-belt, with the exception that they are joined by the top fabric of the belt. These belts can be used with the standard V-belt sheaves, making selection and installation easy.
Positive-drive belts are sometimes called timing belts because they are often used in operations when timing a piece of equipment is critical. However, they are also used in applications where heavy loads cause standard V-belts to slip They are flexible and provide the same benefit as standard V-belts, but their alignment is more critical.
Sheaves are made of cast steel for heavy-duty applications. For lighter applications, they are forged out of steel plate. Cast-iron sheaves are always used in applications where fluctuating loads are present. They provide a flywheel effect that minimizes the effects of fluctuating loads.
When they are mounted to a shaft, sheaves should be straight and have little or no wobble. For drives where the belt enters the sheave at an angle, deep- groove sheaves are available. These are especially useful when the belts must turn or twist.
Deep-groove sheaves can be used anywhere belt stability is a problem. In some cases, one drive shaft drives more than one driven shaft. When this occurs, more than one sheave can be mounted on one shaft. This is necessary only when sheaves of more than one size are needed. If the drive sheaves are the same size, one multibelt sheave can be used.
Most sheaves are balanced and capable of belt speeds of 6,000 feet per minute or less. If you note excessive vibration during operation or excessive bearing wear, you may need to balance or replace the sheaves.
The size of the sheaves in a belt drive system determines the speed relationship between the drive and driven sheaves. For example, if the drive sheave has the same size sheave as the driven, then the speed will be equal.
If we change the size of the driven sheave, then the speed of the shaft will also change. We know what the speed is of the electric motor and the size of the sheaves, and now we can calculate the speed of the driven shaft by using the illustrated formula.
Have class work formula from example.
= 6 x 1800/12
Try other examples
Using the same formula that was used to calculate shaft speed we only switch the location of the driven shaft speed and the driven sheave diameter.
We can now calculate the driven sheave diameter.
Have class work out example:
12 = 6 x 1800/900
Try other examples.
Many times when a mechanic has to change out belts, the numbers on the belts cannot be read. So what should be done? Take a tape measure and wrap it around the sheaves to get the belt length? This is not a very accurate way to determine the length. So, usually the mechanic ends up taking a number of different size belts hoping to have a size that will fit.
Instead, take a couple of measurements, then use a simple formula to calculate the actual length that is needed. First, move the sheaves together until they are as close as the adjustments will allow. Then move the motor or drive out of its travel. Now you are ready to take the measurements. The following information is needed for an equation to find belt length
Diameter of the drive sheave.
Diameter of the driven sheave.
Center-to-center distance between the shafts.
Have class work out example:
Or 98 inches
When calculating multiple sheave systems, think of each set of sheaves as a two-sheave system. Try to solve the following problem by only calculating two sheaves at a time.
In order to calculate the speed of a belt in feet per minute (FPM), the following information is needed:
The diameter of the sheave that the belt is riding on.
The shaft rpm of the sheave.
With this information, we can use the illustrated formula :
Routine maintenance is essential if a belt drive is to operate properly. Belt maintenance should include regular checks of belt alignment and tension. You should also perform frequent inspections of the sheaves and shafts.
Routine maintenance will extend the life of the sheaves and belts. Belt-drive maintenance requires little time, but it must be done regularly. Keeping the belts clean and free of oil and grease will help ensure long belt life
Never force a V-belt onto a sheave. There have been a number of injuries to fingers and hands as a result of this. It can also break the inner strings of the belt leading to premature failure.
The belt should never ride in the bottom of the sheave. The sheave is deeper than the belt. The belt is made to ride near the top of the sheave. The belt may wear to the point that it is riding on the bottom of the sheave. If so, it will slip no matter how much tension is applied to the belt.
Keep used belt sets together for use on multibelt drives.
Routine preventive maintenance is essential if a belt drive is to operate properly.
Belt maintenance should include regular checks of:
belt condition
belt alignment
tension
You should also perform frequent inspections of the sheaves and shafts.
You may need to replace belts that are worn or damaged from overheating or contact with oil or grease.
Never replace one belt of a multibelt drive.
Belts stretch with use.
If you replace one belt of a multibelt drive, it will be tighter than the others
Automotive belts start with either 4L (12.5mm wide) or 3L (9.5mm). The number following it is the outside length of the belt in tenths of inches. The inside length of the belt is typically 2" less for a 4L belt, and 1-1/2" less for a 3L belt. An example would be 4L460, which would be 46" long outside, 44" inside.
Classic" v-belt numbers start with a letter identifying the cross section, A through E - see v-belt dimensions below. A series belts are the most common. The number following it is the inside length in inches. The outside length is typically 2 inches more. An example would be A44, 44" long on the inside, 46" outside; The equivalent of the 4L460 above.
Deliver more horsepower and last longer than conventional belts...
Fully notched cogs for maximum flexibility.
High coefficient rubber edge.
Oil resistant and static conducting.
Proven energy-saving design.
Outlasts conventional belts.
Fewer belts required - drive weight is reduced.
Oil resistant and static conducting
Permits compact, lighter weight drives
High-strength tension member delivers maximum power with minimum stretch
Built for long-term dimensional stability
Molded cog construction under 200-inch belt length
Cable cord envelope construction
Cool running and flexible strong tensile cords minimize stretch
Static conducting and heat and oil resistant
More tolerant of shock loads
Noise can be caused by the slapping of belts against drive guard or other obstruction. Check for improperly installed guard.
Squealing of belts while running is usually caused by poorly tensioned drive or by build up of foreign material in sheave grooves. It can also be caused by oil or grease between belt and sheave groove.
It is important to look at the tight side of a drive to be sure all the belts are running tight. If one or more belts are running loose, the drive needs to be re-tensioned. This could be caused by uneven wear of the grooves in the sheave.
Maintaining correct tension is the most important rule of v-belt care. Belts too loose will slip causing excessive belt and sheave wear. Belts that sag too much are snapped light suddenly when the motor starts and when peak loads occur. This could actually break the belt.
Worn or damaged sheaves
Belts rubbing guard
Sheaves misaligned
Insufficient tension
Wrong belt cross-section or type
Improper or prolonged storage
Excessive heat
Excessive oil or grease
Use of belt dressing
Abrasive environment
Excessive moisture
Inappropriate sheave material
Improper tensioned idler
Belts pried on or misplaced slack
Defective or worn backside idler
Excessive Exposure to Oil Or Grease
Use Of Belt Dressing
Worn or damaged sheaves
Insufficient tension
Wrong belt cross-section or type
Excessive oil or grease
Excessive moisture
Overload drive-Underbelting
Insufficient wrap on small sheave
Belt cover split
Belts pried on or misplaced slack
Foreign objects in grooves
Excessive heat
Sheaves too small
Undersized backside idler
Improperly positioned backside idler
Sheaves misaligned
Improper or prolonged storage
Keep all sheave grooves smooth and uniform.
Burrs and rough spots along the rim can damages belts.
Dust, oil and other foreign matter can lead to pitting and rust.
If sheave sidewalls are permitted to dish out the bottom should ruins belts quickly by chewing off their bottom corners. Also, the belt’s wedging action is reduced and it loses gripping power.
A shiny groove bottom indicates that either the belt or sheave are badly worn and the belt is bottoming in the groove.
Badly worn grooves cause belts to ride lower than the rest of the belts and the effect is similar to mismatched belts. The belts riding high in the grooves travel faster than belts riding low.
Check alignment of drive. Sheaves not aligned properly cause excessive belt and sheave wear.
Check sheave wear using a sheave gauge
Maintaining correction tension is the most important rule of V-belt care. It will give belts 50 % to 100 % longer life.
Belts that are too loose will slip, causing excessive belt and sheave wear.
Make sure drive shafts are parallel.
Where shafts are not parallel, belts on one side are drawn tighter and pull more than their share of the load.
If misalignment is in the sheave, belts will enter and leave the grooves at an angle causing excessive wear.
The shafts must be parallel or the life of the belt will be shortened. The first step is to level the shafts; this is done by placing a level on each of the shafts. Then shim the low side until the shaft is level.
Next, make sure the shafts are parallel. This is done by measuring at different points on the shaft and adjusting the shafts until they are an equal distance apart. Make sure that the shafts are pulled in as close as possible before performing this procedure.
Basically, any degree of misalignment, angular or parallel, will decrease the normal service life of a belt drive.
Angular misalignment results in accelerated belt/sheave wear and potential belt stability problems with individual V-belts. A related problem, uneven belt and cord loading, results in unequal load sharing within multiple belt drives, and can lead to premature failure. Joined V-belts can suffer tie band separation when operating under misaligned conditions.
Symptoms such as high belt tracking forces, uneven tooth/land wear, edge wear, high noise levels, and potential tensile failure due to uneven cord loading are typical indicators of misalignment.
Also, wide synchronous belts are more sensitive to angular misalignment than narrow belts.
Parallel misalignment also results in accelerated belt/sheave wear and potential belt stability problems with individual V-belts. Uneven belt and cord loading is not as significant a concern as with angular misalignment. However, parallel misalignment is typically more of a concern with V-
belts than with synchronous belts. This is because V-belts run in fixed grooves and cannot free float between flanges, as synchronous belts can, to a limited degree.
Parallel misalignment is generally not a critical concern with synchronous belt drives as long as the belt is not trapped or pinched between opposite flanges, and as long as the belt tracks completely
on both sprockets.
Shaft alignment can be checked by measuring the distance between the shafts at three or more locations.
If the distances are equal the shafts will be parallel.
To check the location of the sheaves on the shafts a piece of string can be used.
If properly lined up the string will touch them at the points as indicated below. Rotating each sheave a half revolution will determine if the sheave is wobbly or the drive shaft is bent.
The most important factor in the operation of a v-belt drive is proper belt tensioning.
Belt tension must be sufficient to overcome slipping under maximum load.
To increase total tension, merely increase the center distance. Before attempting to tension any drive it is important that the sheaves be properly installed and aligned.
If a v-belt slips it is too loose, add to the tension by increasing the center distance. Never apply belt dressing.
When you replace a belt, always check the tension immediately after installation, Check the tension again after 24 hours of operation.
Check belt tension using a belt tension gauge.
Always use the manufacturer’s information to set belt tension.
Measure the deflection and tension for the size of the belt.
Check belt tension using a belt tension gauge. Measure the deflection and tension for the size of the belt. (Be sure to write tension and deflection specifications for the mechanic on the PM checklist.) Set tension on belt if deficiency noted.
Risk if the procedure is not followed: MEDIUM. Belt slippage will occur, thus resulting in equipment not operating to operation specifications. Another result from slippage is for belts to break, and the consequences could be a fire or at least machine stoppage.
Identify any type of oil, grease, or chemical within 36 inches of belts (oil leakage from gearbox, motor, bearing, or chemicals from other sources). Write a corrective maintenance work order to repair leak or eliminate source of oil, grease, or chemical from the area.
Risk if the procedure is not followed: HIGH. Belt slippage will occur, thus resulting in equipment not operating to operation specifications. Another result for slippage is for belts to break, and the consequences could he a fire or at least machine stoppage.
Check sheave alignment. If sheaves are not in alignment, align to manufacturer’s specification. (Be sure to write the specification on this procedure; mechanics should not guess on this specification.)
Risk if the procedure is not followed: MEDIUM. Rapid belt wear will occur, thus resulting in equipment not operating to specifications. The belts could break if cords in the belt, begin to break due to this misalignment.
Check sheaves for wear. Use a sheave gauge to ensure the sheave is not worn If worn write a corrective maintenance work order to change sheave at a later date.
Risk if the procedure is not followed: HIGH. Belts will slip (even though you may not hear the slippage), thus resulting in equipment not operating to specifications.
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Basic Rotating Equipment Training
Metal-chain drives are normally used for applications below 3,000 rpm where accuracy and reliability must be greater than that provided by rubber belts. Because a metal chain does not stretch or slip as a belt does, chain drives maintain constant speed ratios under widely varying load conditions and need adjustment infrequently.
Reliability of a chain drive is superior to that of belts, with chain service life typically rated at 10,000 hr, about three to four times longer than that of belts. Also, chain drives generally are more compact than similarly rated belt drives. For example, a 10-hp chain drive is about 50% smaller than a comparable belt drive.
Chains can be replaced more easily than belts because sheave adjustment is not changed for the replacement. Chains can be removed by simply pulling out a master pin in the link and threading the new chain through the drives. But because chain drives do not slip, they cannot provide the overload and jam protection of belt drives. Also, the heavy weight of metal chain compared with light rubber belts produces penalties in efficiency, cost, and response time. Efficiency for chain drives -- about 85 to 90% -- is slightly less than that of conventional belt drives. And chain drives cost about 20 to 50% more than belt drives, with prices ranging from about $300 to $10,000 for 0.70 to 100-hp models.
Chain drives use two types of links: self-forming teeth and extended pin.
Self-forming teeth have slats that extend on each side of the link and conform to the ribbed sides of the sheave faces for a positive engagement. This type usually transmits up to 25 hp and operates at input speeds to almost 900 rpm.
The extended-pin type, with pins projecting on both sides of the link to engage the sheave face, transmits from 7.5 to 100 hp at input speeds to 2,900 rpm. In the range of 7.5 to 25 hp, ratings for the two link types overlap, with prices comparable in the 7.5 to 15-hp range and with extended-pin units being less expensive in the 20 to 25-hp bracket.
Chain drives are used to transmit power between a drive unit and a driven unit.
For example, if we have a gearbox and a contact roll on a conveyor, we need a way to transmit the power from the gearbox to the roll. This can be done easily and efficiently with a chain drive unit.
Chain drives can consist of one or multiple strand chains, depending on the load that the unit must transmit. The chains need to be the matched with the sprocket type and they must be tight enough to prevent slippage. Chain drives can consist of one or multiple strand chains, depending on the load that the unit must transmit. The chains need to be the marched with the sprocket type, and they must be tight enough to prevent slippage.
Chain is sized by the pitch or the center-to-center distance between the pins. This is done in 4” increments, and the pitch number is found on the side bars
Unlike belt drives, chain drives do not slip, therefore there is no power loss due to slippage, which means they are more efficient.
More compact than belt drives. For a given capacity, chain drives are narrower than belt drives, and the sprockets are smaller in diameter than the sheaves on a belt drive.
Chain drives are more practical at slow speeds, yet will operate efficiently at high temperatures.
Chains are generally easier to install than belts.
Chains are not subject to deterioration by oil, grease, or sunlight, and generally withstand chemical and abrasive conditions.
Chains can operate in wet conditions.
Chain stretch under normal operating conditions is slow, and chains require less take-up adjustment than do belts.
Chains are very effective in driving several shafts from one common drive shaft.
Chain drives are generally simpler and less costly than gear drives, and can be used with varying shaft center distances.
Chains are suitable for use on reversing drives.
Chains cannot be used where the drive must slip.
Cannot except much misalignment.
Chains are noisy
May cause vibration within the machinery.
Requires frequent lubrication.
As compared to gear drives, chain drive load capacity is generally smaller and service life is shorter.
Chains are used to transmit power from one rotating shaft to another.
On a typical chain drive, the drive and driven sprockets rotate in the same direction, and maintain a positive speed ratio for the machine.
In most applications, a chain that has an even number of pitches will be used on a sprocket with an odd number of teeth, and a chain with an odd number of pitches will be used on a sprocket with an even number of teeth. This limits developing a wear pattern on the sprockets, by having different links of chain contacting the sprocket teeth on different revolutions, similar to that of using not whole number gearing ratio to limit the wear on gear teeth.
The size of the sprockets in a chain drive system determines the speed relationship between the drive and driven sprockets. For example, if the drive sprocket has the same size sprocket as the driven, then the speed will be equal.
If we change the size of driven sprocket, then the speed of the shaft will also change. If we know what the speed of the electric motor is, and the size of the sprockets, we can calculate the speed of the driven shaft by using the illustrated formula.
Have class work formula from example.
= 6 x 1800/12
Try other examples
Use the formula above to find the chain length.
Chain Length = 6 X .5 + 12 X .5 + (35” x 2) = 75.5”
2 2
In order to calculate the speed of a chain in feet per minute (FPM), we need the following information:
The number of teeth on the sprocket.
The shaft rpm of the sprocket.
The pitch of the chain in inches.
With this information, we can use the illustrated formula :
Roller chains are made up of roller links that are joined with pin links. The links are made up of two side bars, two rollers, and two bushings. The roller reduces the friction between the chain and the sprocket, thereby increasing the life of the unit
Roller chain is sized by the pitch or the center-to-center distance between the pins. This is done in 1/8”increments, and the pitch number is found on the side bars of the chain.
Large pitch conveyor chain has the same basic structure as double pitch conveyor chain but there are some differences. Large pitch conveyor has a headed pin, sometimes a flanged roller (f-roller), and usually does not use a riveted pin. Large pitch conveyor chain is also called engineering class chain.
We see that the chain consists of a set of side-plates, pins (rivets), sleeves and rollers as shown in the diagram.
The diagram shows the construction and major friction/ wear areas.
We can easily see that wear on the pins & the inside of the sleeves will result in chain-slack.
Wear in the roller and outer side of the sleeve will allow the link to settle lower down on the sprocket-tooth. HL is Half-Link.To reduce friction lubrication is required.
Hand Oiling. Apply oil by hand using a hand oiler or a brush, normally at least once everyday. While slowly turning the chain, apply oil evenly 3 to 4 times onto the entire length of the chain. Be careful not to allow hands or clothing to be caught between the chain and the sprocket. When the mechanism is run for the first time after oiling, avoid excess oil splashing over
Drop Lubrication. Oil the chain in a manner such that approximately 5 to 20 drops of oil are applied onto the chain per minute.
Oil Bath. Dip the bottom of the chain approximately 10 mm below the oil level. It is recommended that a simple casing be installed over the chain to prevent oil from splashing over. Use a leak-free oil container. Before installing the oil container, wash it carefully to remove dust, dirt and other foreign particles. Maintain the correct oil level. Do not overfill.
Rotating Plate. The chain is oiled by plate approximately 20 mm below the oil level. The peripheral speed of the plate should be 200 m/min. or faster.
Forced Circulation. Adjust the oil quantity as appropriate to prevent overheating. Use a leak-free oil container. Before installing the oil container, wash it carefully to remove dust, dirt and other foreign particles.
Sprockets are fabricated from a variety of materials; This would depend upon the application of the drive. Large fabricated steel sprockets are manufactured with holes to reduce the weight of the sprocket on the equipment. Because roller chain drives sometimes have restricted spaces for their installation or mounting, the hubs are made in several different styles.
Type A sprockets are flat and have no hub at all. They are usually mounted on flanges or hubs of the device that they are driving. This is accomplished through a series of holes that are either plain or tapered.
Type B sprocket hubs are flush on one side and extend slightly on the other side. The hub is extended to one side to allow the sprocket to be fitted close to the machinery that it is being mounted on. This eliminates a large overhung load on the bearings of the equipment.
Type C sprockets are extended on both sides of the plate surface. They are usually used on the driven sprocket where the pitch diameter is larger and where there is more weight to support on the shaft. Remember this: the larger the load is, the larger the hub should be.
Type D sprockets use a type A or B sprocket mounted on a solid or split hub. The sprocket is split and bolted to the hub. This is done for ease of removal and not practicality. It allows the speed ratio to be changed easily by simply unbolting the sprocket and changing it without having the remove bearings or other equipment.
Refer to previous slide for profiles.
Note: have class identify the class types in the slide.
The slack on the chain should be located on the bottom strand.
It is desirable that a chain wrap of 120 degrees is achieved, but under no circumstance should the chain wrap be less than 90 degrees.
In vertical chain drive arrangements, the chain will tend to hang off the lower sprocket.
This is especially true if the driver sprocket is at the lower position.
Therefore, it is desirable that the shaft centers be arranged such that they are at least 20 degrees off true vertical.
When the proper procedures are followed for installing chains, they will yield years of trouble-free service.
The shafts must be parallel or the life of the chain will be shortened.
The first step is to level the shafts.
This is done by placing a level on each of the shafts, then shimming the low side until the shaft is level.
The next step is to make sure that the shafts are parallel.
Measuring at different points on the shaft, and adjusting the shafts until they are an equal distance apart does this.
Make sure that the shafts are pulled in as close as possible before performing this procedure.
The jacking bolts can be used to move the shafts apart evenly after the chain is installed.
Before installing a set of used sprockets verify the size and condition of the sprockets.
Install the sprockets on the shafts following the manufacturer’s recommendations.
Locate and install the first sprocket.
Then use a straightedge or a string to line the other one up with the one previously installed.
Install the chain on the sprockets, then begin increasing the distance between the sprockets by turning the jacking bolts; Do this until the chain is snug but not tight. To set the proper chain sag deflect the chain 1/4” per foot of span between the shafts. Use a string or straightedge and place it across the top of the chain. Then push down on the chain just enough to remove the slack. Use a tape measure to measure the amount of sag.
Many times when a mechanic has to change out chains there is no way of knowing how long the chain should be. One way is to lay the new chain down beside the old chain, but remember that the old chain has been stretched.
Or, maybe you are installing a new drive and you want to have the chain made up before you install it. So what do you do? One method is to take a tape measure and wrap it around the sprockets to get the chain length.
However, this is not a very accurate way to determine the length. Instead, let’s take a couple of measurements, then use a simple formula to calculate the actual length that is needed.
First, move the sprockets together until they are as close as the adjustments will allow. Then move the motor or drive out 1/4 of its travel. Now we are ready to take our measurements. The following information is needed for an equation to find the chain length:
1. Number of teeth on the drive sprocket.
2. Number of teeth on the driven sprocket.
3. Center-to-center distance between the shafts.
4. The chain pitch in inches.
Chain cleanliness and proper lubrication are vital to your chains long life. Foreign material left on the chain can have an abrasive effect when mixed with the lubrication and cause excessive wear.
Check for evidence of wear. If the inside surfaces of roller chain link sidebars appear to be worn, then the drive is probably misaligned, and the alignment should be checked. This type of problem will also result in wear on the side of the sprocket teeth.
Inspect chain for flexibility. Stiff chain joints can be a result of dirt or grit in the rollers.
Inspect the amount of chain stretch or elongation. A single pitch roller chain should be replaced if the amount of stretch is equal to or greater than 3 percent of its original length.
Check for any signs of physical damage to the chain, such as broken or cracked parts, loose pins and bushings, or indications of corrosion.
Check the chain for elongation by examining the conveyor operation at the sprockets. A worn or elongated chain will ride high on the sprocket teeth .
The side of the sprocket teeth should be inspected for signs of wear, indicating a possible alignment problem.
Check the teeth for signs of wear, indicated by a “hooked” shaped. This is normal wear, which may have been accelerated by a loose fitting chain. In some cases it is possible to turn the sprocket around, and extend its life.
Inspect for signs of physical damage to the sprocket, such as broken or chipped teeth, or excessive corrosion.
Check the sprocket run out on the shaft, and inspect the keys and keyways for wear or damage.
The use of a worn drive chain causes the sprocket teeth to wear. The wear is evident by scratches, groove patterns, polished surfaces, galling or visible changes in tooth formation .
Replace sprockets when worn teeth show physical changes such as visible tooth deformation, wear patterns and other tooth deformation that affects operation. Typically, the chains and sprockets are replaced as a unit.
Inspect a chain for wear by inspecting the links for worn bushings. If worn bushings are noted, write a corrective maintenance work order so that the replacement can be planned and scheduled at a later time.
Risk [the procedure is not followed: HIGH. Chain breakage will occur.
Lubricate chain with lightweight oil recommended by chain manufacturer. (Ask your chain supplier to visit your site and make recommendations based on documentation they can present to you.)
Risk if the procedure is not followed: HIGH. Chain breakage will occur.
Check chain sag. Measure the chain sag using a straight edge or string and measure the specifications noted on this PM task. (The chain sag specification can be provided by your chain supplier, or you can use the procedure noted earlier in this chapter.) WARNING: The specification must be noted on the PM procedure.
Set tension, and make a note at the bottom of the PM work order, if a deficiency is noted.
Risk if the procedure is not followed: MEDIUM. Sprocket and chain wear will accelerate, thus causing equipment stoppage.
Inspect sprockets for worn teeth and abnormal wear on the sides of the sprockets. (The question is: Can the sprockets and chain last for two more weeks without equipment stoppage?) If the sprockets and chain can last two weeks then write a corrective maintenance work order in order for this job to he planned and scheduled with the correct parts. If the sprocket cannot last two weeks, then change all sprockets and the chain. Set and check sheave and chain alignment and tension. WARNING: When changing a sprocket, all sprockets, and the chain, should be changed because the difference between a worn and new sprocket in pitch diameter can be extreme, thus causing premature failure of the sprockets and chain.
Risk if the procedure is not followed: High. Worn sprockets are an indication of the equipment being in a failure mode. Action must be taken.
