Quick return mechanism

1. Return Mechanism
Experiment # 1
To study working of quick return mechanism
Apparatus:
Quick return mechanism, Shaper machine mechanism
Figure:
Figure 1: Quick Return Mechanism
Figure 2: Shaping Machine mechanism
Theory:
Mechanism:
When one of the links of a kinematics chain is fixed, the chain is known as mechanisms. It may
be used for transmitting or transforming motion e.g. engine indicators, typewriter etc.
Types:
a) Simple mechanism
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2. Return Mechanism
b) Compound mechanism
a) Simple Mechanism:
A mechanism with four links is known as simple mechanism.
b) Compound Mechanism:
The mechanism with more than four links is known as compound mechanism.
Machine:
When a mechanism is required to transmit power or to some particular type of work; it then
becomes a machine.
In such cases the links is to be designed to withstand the force (both static and kinetic safely)
A little consideration will show that a mechanism may be regarded as a machine in which each
part is reduced to the simplest form to transmit the required motion.
Inversion:
Sometimes, one of the links in a kinematic chain is fixed. In such a case, we may obtain as many
mechanisms as the links of the kinematic chain. This method of obtaining different mechanisms,
by fixing in turns different links in a kinematic chain, is known as inversion.
Types of kinematic chains:
The most important kinematic chains are those which consist of four lower pairs, each pair being
a sliding pair or a turning pair. The following types of kinematic chain are important from subject
point of view:
a. Four bar chain or quadratic cycle chain
1. Beam engine
2. Coupling rod of locomotive
3. Watt’s indicator mechanism
b. Single slider crank chain
1. Pendulum pump
2. Oscillating cylinder engine
3. Quick return motion mechanism
4. Rotary I.C. engine (Gnome engine)
c. Double slider crank chain
Four bar chain:
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3. Return Mechanism
A kinematic chain which consists of four links is known as four bar chain. In four bar chain each
of four pairs is a turning pair. These links may or may not have same length.
If link AB is fixed and a small displacement is given to link AD the resulting displacement of
link BC and CD are also definite. So, this shows that the relative motion is completely
constrained. The structure of simple kinematic chains is derived from simple four bar chain.
Sometimes a link is added which will form a turning pair with existing link and also themselves
form a pair.
Compound kinematic chain:
If a kinematic chain has more than four pairs it is known as Compound kinematic chain.
Inversions of four bar chain:
The practical application of a four bar chain are found in:
1. Beam engine (Crank and lever mechanism)
2. Coupling rod of locomotive (Double crank mechanism)
3. Watt’s indicator mechanism (Double lever mechanism)
1. Beam engine:
A part of the mechanism is shown in the figure. It consists of four links and is also known as
crank and lever mechanism.
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4. Return Mechanism
In this mechanism the lever oscillates about a fixed centre D while crank rotates about a fixed
centre A.
The end E of lever CDE is connected to a piston rod, which reciprocates due to rotation of the
crank. In other words the purpose of this mechanism is to convert rotatory motion to
reciprocating motion.
2. Coupling rod of locomotive:
The mechanism of a coupling rod of locomotive consists of four links as shown in figure:
In this mechanism, the links AD and BC acts as cranks connected to the respective wheels and
the link CD acts as a coupling rod. The link AB is fixed in order to maintain a constant centre to
centre distance between them.
3. Watt’s indicator mechanism:
A watt’s indicator mechanism which consists of four links is shown in figure:
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5. Return Mechanism
The four links are:
 Fixed link at A
 Link AC
 Link CE
 Link BFD
It may be noted that BD and FD form as one link because these two parts have no relative
motion between them. The links CE and BFD acts as levers. On any small displacement of the
mechanism the tracing point E traces out approximately a straight line. The initial position of
mechanism is shown by solid lines whereas dotted lines show the position of mechanism when
gas or steam pressure acts on the indicator.
Single slider crank chain:
A slider crank chain consists of one sliding pair and three turning pairs. It is, usually, found in
reciprocating steam engine mechanism. These types of mechanisms convert rotary motion into
reciprocating motion and vice versa. In a single slider crank chain, as shown in fig. P1, P2, and P3
are called turning pairs while P4 is sliding pair.
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6. Return Mechanism
The link 1 corresponds to the frame of engine, which is fixed. The link 2 corresponds to the
crank; link 3 corresponds to cross head. As the crank rotates, the cross head reciprocates in the
guides.
Inversions of slider crank chain:
Slider crank is a 4-link mechanism. We know that by fixing, in turn, different links in a kinematic
links in a kinematic chain, an inversion is obtained and we can obtain as many mechanisms as
the links in the kinematic chain. It is thus obvious, that four inversions are possible. These
inversions are in following mechanism.
1. Pendulum Pump or Bull Engine
2. Oscillating Cylinder Engine
3. Crank and Slotted Lever Quick Return Motion Mechanism
4. Whitworth’s quick return mechanism
5. Quick Return I.C. Engine
1. Pendulum pump or bull engine:
In this mechanism is obtained by fixing the cylinder or link as shown in fig.
In this case when link 2 rotates, the piston in link 1 reciprocates and link 3 oscillates. This
mechanism is of little importance from practical point of view.
2. Oscillating Cylinder Engine:
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7. Return Mechanism
The arrangement of oscillating cylinder engine mechanism as shown:
In this type of mechanism, link 3 is fixed. As link 2 (crank) rotates, link 1 reciprocates and link 4
oscillates, which is pivoted to link fixed at A.
The link 3 corresponds to connecting rod in reciprocating engine mechanism.
3. Crank and slotted lever quick return motion mechanism:
This mechanism is mostly used in shaping machine, slotting machine and in rotary internal
combustion engine. The mechanism consists of a fixed link AC as shown in fig.
The driving crank revolves with uniform angular speed about the fixed center C.A sliding block
attached to the crank pin at E slides along the slotted bar AP and thus causes AP to oscillate about
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8. Return Mechanism
the fixed center A. A short link bar PR transmit the motion from AP to the ram which carries the
tool and reciprocates along the line of stroke R 1 R2. The line of stroke of the ram (R 1 R2) is
perpendicular to AO produced. In the extreme position, AP and AP2 are tangential to the circle
and the cutting tool is at the end of the stroke. The cutting stroke occurs when the crank rotates
from the position CB1 to CB2 (or through an angle beta) in the clockwise direction. The return
stroke occurs when the crank rotates from the position CB2to CB1 (or through an angle alpha) in
the clockwise direction.
4. Whitworth’s quick return mechanism:
This mechanism is mostly used in shaping machine; slotting machine. It is obtained by fixing the
link 2 as shown in fig 5.14. In this mechanism, the driving crank CA (link 3) rotates at uniform
angular speed. The slider (link 4) slides in a slot in a slotted lever PA (link 1) which is pivoted a
fixed point D. The connecting rood PR carries the ram at R, to which a cutting tool is fixed. The
motion of the tool is constrained along the line RD produced i.e., along a line passing through D
and perpendicular to CD.
5. Rotary Internal Combustion Engine or Gnome Engine:
Sometimes back rotary internal combustion engines were used in aviation. But now days gas
turbines are used in its place. It consists of seven cylinders in one plane and all revolves about
fixed center D as shown in fig. while crank (link 2) is fixed. In this mechanism, when connecting
rod (link 4) rotates, the piston (link 3) reciprocates inside the cylinder link forming 1.
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9. Return Mechanism
Working of Quick Return Mechanism:
A quick return mechanism such as the one seen below is used where there is a need to convert
rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and
backwards. Many machines have this type of mechanism and in the school workshop the best
example is the shaping machine.
Purpose:
It is often desirable to reduce the non-productive time in machining so we use quick return
mechanism to let the backward stroke operate at a higher speed than forward stroke. [1]
Applications:
Presses:
The Whitworth quick return mechanism has been modified and used for constructing high-
velocity impacting press. The impacting press drive consists of a Whitworth quick return
mechanism consisting of a crank and a drive arm together with a variable speed d.c. motor, a
flywheel, bearings, etc. The end of the drive arm is attached by a connecting rod to a cycloidal
cam. In single cycle operation, the cam is made to engage with an upper platen (or ram) which
impacts the workpiece. The upper platen and cam are both mounted on multirod supports with
linear ball bushings. A brake is provided on the flywheel for emergency purposes. The high-
speed impacting presses are subject to severe dynamic forces when operating at speeds of 200
rpm and greater. Although the mechanism has been designed to withstand operation at higher
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10. Return Mechanism
frequencies, the loads transmitted to the building foundations cause unwanted vibrations and
noise. [2]
Shaper machine:
Shaper consists of a hollow machine bed made of cast iron which rests on the ground. Inside the
hollow portion the machine drive mechanism is housed. This mechanism is called slotted lever
quick return mechanism and it drives a horizontal ram which reciprocates in the guide ways
provided on the top surface of the machine frame.
Since useful work is done only during the forward stroke of ram, the mechanism driving the ram
is so designed that the return stroke is completed in much less time than the forward stroke. The
slotted lever quick return mechanism is shown in Figs. 2.2(a) and 2.2(b).
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11. Return Mechanism
The crank AB (of adjustable length R) rotates with a uniform angular speed. The crank pin B is
in the shape of a die block which is free to slide inside the slot in the slotted lever OBC. This
slotted lever is pivoted at O and the other end C is connected to the ram by a short link arm as
shown in Fig. 2.2 (a). When the crank AB rotates clockwise from position AB1to AB2, the ram
moves forward from left to right and when it rotates from position AB2to AB1the ram returns
back to its original position. Clearly the time taken to complete forward stroke is proportional to
angle α (refer to Fig. 2.2 (b)) and the return stroke is completed in less time which is proportional
to angle β.[3]
Procedure:
 Take quick return mechanism.
 Rotate crank 10 degree to get the slider reading
 Repeat the experiment for36 readings
 After taking all reading draw displacement graph.
 Take shaper mechanism.
 Rotate crank 10 degree to get the slider reading
 Repeat the experiment for 36 readings
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12. Return Mechanism
 After taking all reading draw displacement graph.
COMMENTS:
 Quick-return mechanisms are used in shapers,
power-driven saws, and many other applications where a
slower working stroke in comparison to return stroke is
desired
 Readings might not be accurate due to friction in the
apparatus
 In quick return mechanism
maximum value is achieved
before slider crank.
 Maximum value at 100 degrees
is 114 mm and at 110 is 1.95 in.
PRECAUTIONS:
 The observer must stand in front
of apparatus while taking
readings.
 Two persons should take one
reading to avoid any kind of
error.
Observations and calculations:
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(deg.)Ө d (mm)
0 0.55
10 0.35
20 0.1
30 0.2
40 0.5
50 0.75
60 1.05
70 1.3
80 1.55
90 1.7
100 1.9
110 1.95
120 1.9
130 1.8
140 1.45
150 1
160 0.4
170 0.3
180 1.8
190 1.3
200 1.7
210 1.9
220 2.1
230 2.2
240 2.3
250 2.29
260 2.27
270 2.25
280 2.15
290 2
300 1.9
310 1.7
320 1.565
330 1.35
340 1.1
350 0.9
360 0.6
(deg.)Ө d (mm)
0 61
10 69
20 76
30 83
40 90
50 96
60 102
70 106
80 110
90 113
100 114
110 113
120 112
130 108
140 101
150 93
160 82
170 71
180 58
190 45
200 35
210 25
220 17
230 11
240 7
250 5
260 5
270 7
280 9
290 13
300 19
310 24
320 31
330 38
340 45
350 52
360 61
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13. Return Mechanism
References:
[1]
R.S Khurmi and J.K Gupta, Theory of Machines
[2]
Surendra N Dwivedi , Mechanism and Machine Theory, Volume 19, Issue 1, 1984, page 51-59
[3]
Mechanicalengineeringstudymaterial.wordpress.com
Lab report 13
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