- 1. Unit 4 Power Screws Prepared By Prof. M.C. Shinde [9970160753] Mech. Engg. Dept., JSCOE, Hadapsar
- 2. Unit 4 Power Screws Session 4.1 Introduction to Power Screws , Terminology, Forms of Thread Prepared By Prof. M.C. Shinde Mech. Engg. Dept., JSCOE, Hadapsar
- 3. SPPU Syllabus Content: (06 hrs) Forms of threads, multiple start screws, Torque analysis and Design of power screws with square and trapezoidal threads, Self locking screw, Collar friction torque, Stresses in power screws, design of a C-Clamp. Design of screw jack, Differential and Compound Screw and Re- circulating Ball Screw (Theoretical treatment only).
- 4. Power Screw •A power screw is a mechanical device used for converting rotary motion into linear motion and transmitting power. •A power screw is also called a translation screw. It uses helical translatory motion of the screw thread in transmitting power rather than clamping the machine components.
- 5. Power Screw •There are three essential parts of the power screw, viz., screw, nut and a part to hold either the screw or the nut in its place. •Depending upon the holding arrangement, power screws operate in two different ways. •In some cases, the screw rotates in its bearing, while the nut has axial motion. The lead screw of the lathe is an example of this category. •In other applications, the nut is kept stationary and the screw moves in an axial direction. A screw jack and machine vice are the examples of this category.
- 6. The main applications of power screws are as follows: (i) To raise the load, e.g., screw-jack; (ii) To obtain accurate motion in machining operations, e.g., lead-screw of lathe;
- 7. The main applications of power screws are as follows: (iii) To clamp a work piece, e.g., a vice; (iv) To load a specimen, e.g., universal testing machine.
- 8. Advantages of Power Screw • A power screw has large load carrying capacity. • The overall dimensions of the power screw are small, resulting in compact construction. • A power screw is simple to design. • The manufacturing of a power screw is easy without requiring specialised machinery. Square threads are turned on the lathe. Trapezoidal threads are manufactured on a thread milling machine. • A power screw provides large mechanical advantage. A load of 15 kN can be raised by applying an effort as small as 400 N. Therefore, most of the power screws used in various applications like screw-jacks, clamps, valves and vices are manually operated.
- 9. Advantages of Power Screw •A power screw provides precisely controlled and highly accurate linear motion required in machine tool applications. •A power screw gives smooth and noiseless service without any maintenance. •There are few parts in a power screw. This reduces cost and increases reliability. •A power screw can be designed with self locking property. In screw-jack application, self-locking characteristic is required to prevent the load from descending on its own.
- 10. Disadvantages of Power Screw •A power screw has very poor efficiency, as low as 40%. Therefore, it is not used in continuous power transmission in machine tools, with the exception of the lead screw. •High friction in threads causes rapid wear of the screw or the nut. Therefore, wear is a serious problem in power screws.
- 11. Terminology of Power Screw •Nominal diameter(d) •Core diameter(dc) •Mean diameter(dm) •Pitch (p) •Lead (l) •Lead angle(ƛ) •Hand of threads
- 12. Nominal diameter(d) •It is the largest diameter of an external or internal thread. •The screw is specified by this diameter.
- 13. Core diameter(dc) •It is the smallest diameter of an external or internal thread.
- 14. Pitch (p) •It is the distance from any point on the thread to the corresponding point on the adjacent thread measured parallel to the axis.
- 15. Lead (l) “It is the distance which a screw advances axially in one rotation of the nut”. OR “distance between two corresponding points on the same helix. Lead=number of starts*pitch L=N*p For single start screw pitch is equal to lead……so on.
- 16. Lead angle(ƛ) “It is an angle made by a helix or thread with plane perpendicular to an axis of screw.”
- 17. Hand of threads When the axis of screw is vertical if the thread slope upward from left to right, it is Right Hand Threads. Right Hand Threads. Left Hand Threads.
- 18. Forms of threads
- 19. 1.Square Threads • Advantages 1. Square threads have maximum efficiency of all thread forms. 2. They exert minimum radial pressure on nut. 3. They can transmit power in either direction. • Disadvantages 1. Strength of the square threads is lowest of all the thread forms. 2. Theses threads cannot be used conveniently with split nut because: engagement and disengagement is difficult • Applications: Used in Screw jacks, presses & clamping devices
- 20. 2.ACME/Trapezoidal Threads • Advantages 1. Acme threads permit the use of split nut which can compensate the wear. 2. Acme threads are stronger than the square threads in shear because of the larger cross-section at the root. 3. Acme threads can transmit power in either direction. • Disadvantages 1. Because of slope given to the sides the efficiency of acme threads is lower than that of square threads. 2. Slope on the sides introduces some bursting pressure on the nut. • Applications: Used for lead screws of machine tools, bench vices
- 21. 3. Buttress Threads • Advantages 1. Buttress threads are stronger in shear than any other power threads because of the largest cross section at the root. 2. Buttress threads combine the high efficiency of square threads and high strength of V- threads. • Disadvantages 1. Buttress threads are used to transmit power in only one direction. 2. Theses threads are difficult to manufacture. • Applications: Used in screw jacks & vices where force is to be applied in only one direction.
- 22. 1.Write short note on Power Screws. 2.Explain in brief terminology used in Power Screws. 3.Explain Different types of threads. Assignment 4.1
- 23. Unit 4 Power Screws Session 4.2 Torque Analysis, Self Locking &Overhauling of Screw Prepared By Prof. M.C. Shinde Mech. Engg. Dept., JSCOE, Hadapsar
- 24. Torque required to raise the load against thread friction • The advancement (motion) of the screw or nut in the direction of load is equivalent to raising the load, as shown in fig.1 the force diagram of an equivalent inclined plane for raising the load is shown in fig.2
- 25. Torque required to raise the load against thread friction
- 26. Torque required to raise the load against thread friction
- 27. Torque required to raise the load against thread friction
- 28. Torque required to lower the load against thread friction • The advancement (motion) of the screw or nut in the direction of load is equivalent to lowering the load, as shown in fig.1 the force diagram of an equivalent inclined plane for raising the load is shown in fig.2
- 29. Torque required to lower the load against thread friction
- 30. Torque required to lower the load against thread friction
- 31. Torque required to lower the load against thread friction
- 32. Self locking Screws Torque required to lower the load against thread friction is given by In this equation if the torque required to lower the load Tt will be positive. Such screw is known as self-locking screw. For self-locking screw, friction angle is greater than lead angle and torque required to lower the load Tt will be always positive. Applications: Self locking screw is used in screw-jack & C-Clamps.
- 33. Over hauling Screws Torque required to lower the load against thread friction is given by In this equation if the torque required to lower the load Tt will be negative. i.e. load will start moving downward without the application of any torque causing the screw to rotate. Such screw is known as over hauling screw. For over hauling screw, friction angle is less than or equal to lead angle and torque required to lower the load Tt will be zero or negative.
- 34. 1.Write short note on Self locking of screw 2.Write short note on Over hauling of screw 3. Derive expression for torque required to raise the load. Assignment 4.2
- 35. Unit 4 Power Screws Session 4.3 Screw Efficiency, Collar Friction Torque Prepared By Prof. M.C. Shinde Mech. Engg. Dept., JSCOE, Hadapsar
- 36. Screw efficiency of square threads Screw efficiency: it is the ratio of zero friction input torque to the actual input torque Expression for Screw efficiency: zero friction input torque to the actual input torque is given by,
- 37. Collar friction Torque •In many applications, load does not rotate with screw, and hence some additional torque must be applied to overcome the friction at collar. •Fig. shows the power screw with the collar and cup. The collar of the power rotates with screw while cup remains stationary due to load W. this results in friction at the annular surface between the collar and the cup.
- 38. Expression for collar friction Torque •The torque required to overcome the collar friction is given by, According to uniform pressure theory According to uniform wear theory In general can be written in this form
- 39. Overall efficiency of Power screw- Method 1 •Overall efficiency :- ratio of total zero friction input torque to total actual input torque. Expression for overall efficiency Total actual input torque is given by Total zero friction input torque is given by overall efficiency
- 40. Overall efficiency of Power screw- Method 2 •When torque T completes one rotation(i.e. rotates through radians) load W moves through a distance l. Expression for overall efficiency Work output = Work input = overall efficiency is given by
- 41. Ex.4.1 The following data refers to a screw jack: • Nominal diameter of screw=40mm • Pitch of threads=7mm • Type of screw=single start square threaded • Coefficient of thread friction=0.15 • Coefficient of collar friction=0.1 • Effective mean diameter of collar=70m, if operator can comfortably exert a force of 150N at a radius of 1.2m to raise the load, calculate i) The maximum load that can be lifted ii) The efficiency of the screw iii) The overall efficiency.
- 42. •Given:
- 43. To find Lead angle
- 44. To find friction angle
- 45. Torque required to raise the load
- 46. Torque applied by the operator
- 47. Maximum load that can be lifted Torque reqd to raise the load=torque applied by operator
- 48. Screw efficiency
- 50. Ex.4.2 in a machine tool application, the tool holder is pulled by means of an operating nut mounted on a screw. the tool holder travels at a speed of 5m/min. the screw has a single start square threads of 48mm nominal diameter and 8 mm pitch. The operating nut exerts a force of 500N to drive the tool holder. The mean radius of friction collar is 40 mm. if the coefficient of friction for thread and collar surfaces is 0.15 calculate: i) The power required to drive the screw ii) The efficiency of the mechanism
- 51. Given
- 52. To find Lead angle
- 53. To find friction angle
- 54. Torque required to drive the screw
- 55. Speed of screw
- 56. Power required to drive the screw
- 58. Ex.4.3 a two start, trapezoidal screw is used in a screw jack to raise a load of 300 N. the screw has a nominal diameter of 100mm and a pitch of 12mm. The coefficient of screw friction is 0.15. neglecting the collar friction, determine : i) The torque required to raise the load; ii) The torque required to lower the load; iii) Screw efficiency
- 59. Given
- 60. To find Lead angle
- 61. To find friction angle
- 62. Torque required to raise the load
- 63. Torque required to lower the load
- 64. Screw Efficiency
- 65. Ex.4.4 a machine vice has a single start square threaded screw with a nominal diameter of 22 mm and pitch of 5 mm. a clamping collar has inner and outer diameter as 45 mm and 55mm respectively. The coefficient of friction for threads as well as collar is 0.15. the operator can apply a force of 100 N on which is 150 mm long. Assuming uniform wear condition for collar, determine ; i) Clamping force developed ii) Overall efficiency
- 66. Given
- 67. To find Lead angle
- 68. To find friction angle
- 69. Torque required to overcome thread friction
- 70. Torque required to overcome the collar friction
- 72. Torque applied by the operator
- 73. Maximum load that can be lifted Torque reqd to overcome friction=torque applied by operator
- 75. Ex.4.5 the lead screw of a lathe has a single start I.S.O. metric trapezoidal threads of 52mm nominal diameter and 8 mm pitch. The screw is required to exert on axial force of 2kN in order to drive the tool carriage during the turning operation. The thrust is carried on collar of100mm outer diameter and 60 mm inner diameter. The values of coefficient of friction at the screw threads and collars are 0.15 and 0.12 respectively. If the load screw rotates at 30 rpm. Calculate i) The power required to drive the lead screw ii) The efficiency of screw. Evaluate the results using uniform wear theory and uniform pressure theory.
- 76. Given
- 77. To find Lead angle
- 78. To find friction angle
- 79. Case I Uniform Wear theory
- 80. Torque required to drive the screw Note:- Consider Uniform wear theory
- 81. Power required to drive the screw
- 82. Screw efficiency Note:- Consider Uniform wear theory
- 83. Overall efficiency Note:- Consider Uniform wear theory
- 84. Case II Uniform Pressure theory
- 85. Torque required to drive the screw Note:- Consider Uniform Pressure theory
- 86. Power required to drive the screw
- 87. Screw efficiency Note:- Consider Uniform Pressure theory
- 88. Overall efficiency Note:- Consider Uniform Pressure theory
- 89. Stresses in Power Screw 1. Stresses in Screw Body 2. Stresses in Screw Threads
- 90. 1. Stresses in Screw Body
- 91. Direct compressive or tensile stress Direct compressive or tensile stress in a screw body due to an axial force W is given by
- 92. Torsional shear stress Torsional shear stress in a screw body due to twisting moment (or Torque) T is given by
- 93. Maximum shear stress According to maximum shear stress theory, the maximum shear stress induced in the screw body is given by
- 94. Buckling of screw When an axial load on the screw is compressive and the unsupported length of screw between the load and nut is too long, screw body must be checked for the buckling failure. According to J.B. Johnson formula critical or buckling load for the screw is given by Syc - yield strength in compression for screw material N/mm2 E - modulus of elasticity for screw material, N/mm2 C - end fixity coefficient L - Unsupported length of the screw between load & nut K - least radius of gyration of screw cross section
- 95. 2. Stresses in Screw Threads
- 96. Bearing Pressure As there is relative motion between screw and nut ,there exist bearing pressure between contacting surfaces of screw and nut threads. Bearing pressure between threads is given by, Where d-nominal diameter of screw Z-number of threads in engagement h-height of nut h=Z*p
- 97. Direct shear stress in screw threads Direct shear stress induced in the screw threads is given by Where d-nominal diameter of screw dc-core diameter of screw Z-number of threads in engagement t-thickness or width of thread at the root h-height of nut h=Z*p
- 98. Direct shear stress in nut threads Direct shear stress induced in the nut threads is given by Where d-nominal diameter of screw Z-number of threads in engagement t-thickness or width of thread at the root h-height of nut h=Z*p
- 99. Ex.4.6 The construction of a gate valve used in high pressure pipeline is shown in fig. the screw is rotated by means of the handle. The nut is fixed to the gate. When the screw rotates the nut along with gate moves downward or upward depending upon the direction of rotation of the screw. The screw has single start square threads of 40mm outer diameter and 7mm pitch. The weight of the gate is resistance between the gate and its seat. The resultant frictional resistance in axial direction is 2kN. The inner and outer diameters of thrust washer are 40mm and 80mm respectively. The coefficient of friction at the threads and at the washer are 0.15 and 0.12 respectively. If the handle is rotated by two arms, each exerting equal force at radius of 500mm from the axis of the screw,
- 100. calculate: i. The maximum force exerted by each arm when the gate is being raised; ii. The maximum force exerted by each arm when the gate is being lowered; iii. The efficiency of the gate mechanism iv. The number of threads in engagement ,if the permissible bearing pressure is 5 N/mm2 v. Length of nut
- 101. Given
- 102. To find Lead angle
- 103. To find friction angle
- 104. Torque required to raise the gate
- 105. Force to be exerted by each arm to raise the gate(FR)
- 106. Torque required to lower the gate(TL) When the gate is lowered frictional resistance due to water pressure, which always opposes the motion, acts upward. Total force acting in downward direction which is to be lowered is
- 107. Force to be exerted by each arm to lower the gate(FL)
- 108. Efficiency of gate mechanism
- 109. Number of threads in engagement(Z)
- 110. Length of Nut
- 111. Ex.4.7 A power screw having double start square threads of 30mm nominal diameter and 6mm pitch is acted upon by an axial load of 10kN. The outer and inner diameters of screw collar are 50mm and 30mm respectively. The coefficient of thread friction and collar friction may be assumed as 0.25 and 0.18 respectively. The screw rotates at 12 rpm. Assuming uniform wear condition at the collar and allowable thread bearing pressure of 6.3N/mm2,find; 1. The power required to rotate the screw; 2. The stresses in screw; 3. The height of nut.
- 112. Given
- 113. To find Lead angle
- 114. To find friction angle
- 115. Torque required to overcome thread friction
- 116. Torque required to overcome the collar friction
- 118. Power required to rotate the screw
- 119. Direct compressive stress in body
- 120. Torsional shear stress in body
- 121. Direct shear stress in screw threads
- 122. Maximum shear stress in body
- 123. No. of threads in engagement
- 124. Height of Nut
- 125. Ex.4.8 A square threaded, triple start power screw, used in a screw jack has a nominal diameter of 50mm and a pitch of 8mm. The screw jack is used to lift load of 8kN. The coefficient of thread friction is 0.12 and collar friction is negligible. If the length of nut is 48mm, calculate; i. The maximum shear stress in the screw body; ii. The direct shear stress in the screw and nut; iii. The bearing pressure State the conditions of the screw.
- 126. Given
- 127. To find Lead angle
- 128. To find friction angle
- 130. Direct compressive stress in screw body
- 131. Torsional shear stress in screw body
- 132. Maximum shear stress in a screw body
- 133. Direct shear stress in a screw threads
- 134. Direct shear stress in nut threads
- 135. Bearing pressure
- 136. Condition of screw In this case, =9.429 and =6.8428 As screw is over hauling
- 137. Design of Screw Jack
- 138. Design of Screw body
- 141. Bending stress
- 143. Maximum Shear stress
- 144. Design of Nut
- 145. Bearing Pressure
- 146. Shear stress induced in Nut threads
- 147. Shear stress induced in Screw threads
- 148. Design of Handle
- 149. Length of handle
- 150. Diameter of handle
- 151. Ex.4.9 Design a bottle type screw jack for a load capacity of 65kN and a lifting height of 2.5m, with the following data; • Tensile yield strength of screw material(alloy steel 40CrL)= 460 N/mm2 • Compressive yield strength of screw material(alloy steel 40CrL)=550 N/mm2 • Tensile yield strength of nut material(phosphor bronze)=110N/mm2 • Compressive yield strength of nut material(phosphor bronze)=130N/mm2 • Yield strength of nut material in shear(phosphor bronze)=90N/mm2 • Tensile yield strength of handle material (plain carbon steel,55C8)=400N/mm2
- 152. • Permissible bearing pressure between the screw and nut =18N/mm2 • Coefficient of friction between the screw and nut=0.14 • Coefficient of collar friction=0.16 • Factor of safety=3
- 153. Design of Screw body
- 154. To find core diameter dc of screw
- 155. Select standard square thread for screw
- 156. To find Lead angle
- 157. To find friction angle
- 158. Torque required to overcome thread friction
- 159. Dimensions of the collar
- 160. Torque required to overcome the collar friction
- 162. Direct compressive stress in body
- 163. Bending stress
- 164. Torsional shear stress in body
- 165. Maximum Shear stress
- 166. Check maximum torsional shear stress < Permissible Design of screw body is safe hence
- 167. Design of Nut
- 168. Bearing Pressure To find Z, h
- 169. Check Shear stress induced in Nut threads<Permissible Hence nut threads are safe against shear failure
- 170. Check Shear stress induced in Screw threads<Permissible Hence screw threads are safe against shear failure
- 171. Design of Handle
- 172. Length of handle
- 173. Diameter of handle To find dh, H1=2*dh
- 174. Ex.4.10 design a nut of screw jack using following data; • Load to be lifted =50kN • Lift of screw jack=500mm • Pitch of threads=12mm • Tensile yield strength for nut=300MPa • Permissible bearing pressure=12MPa • Factor of safety=5
- 175. Given
- 176. Allowable tensile stress for nut
- 177. Allowable shear stress for nut
- 178. Bearing pressure between nut and screw thread
- 179. Dimensions for Nut let us consider Z=4 threads
- 180. Check direct shear stress in Nut< Permissible Hence nut threads are safe against shear failure
- 181. Design of C Clamp
- 182. Design of Screw body
- 185. Bending stress
- 187. Maximum Shear stress
- 188. Design of Nut
- 189. Bearing Pressure Height of Nut(h) =Z*p
- 190. Shear stress induced in Nut threads
- 191. Shear stress induced in Screw threads
- 192. Design of Handle
- 193. Length of handle
- 194. Ex.4.11 The following data refers to C-clamp; • Maximum clamping force required=4kN • Tensile yield strength of screw material(Plain Carbon Steel,35C8)=320N/mm2 • Compressive yield strength of screw material(Plain Carbon Steel,35C8)=390N/mm2 • Shear strength of the nut and body material(FG200)=230N/mm2 • Coefficient of the screw friction=0.14 • Coefficient of the collar friction=0.16 • Mean collar radius=8mm • Permissible bearing pressure between nut & screw=12N/mm2 • Distance between the axis of the handle and nut surface, in clamped condition=150mm • Force applied by an operator=100N • Distance between the axis of the screw and centroidal axis of vertical column of the C-clamp body=100mm
- 195. • Factor of safety=3 Design the screw and nut for C-clamp and determine the following parameters: i. The standard dimensions of screw body; ii. The height of nut; iii. Length of handle; iv. The dimensions of I-Section of the C-clamp body.
- 196. Design of Screw body
- 197. To find core diameter dc of screw
- 198. Select standard square thread for screw
- 199. To find Lead angle
- 200. To find friction angle
- 201. Torque required to overcome thread friction
- 202. Torque required to overcome the collar friction
- 204. Torsional shear stress in body
- 205. Bending stress in body
- 206. Maximum Shear stress
- 207. Check maximum torsional shear stress < Permissible Design of screw body is safe
- 208. Design of Nut
- 209. Bearing Pressure
- 210. Height of Nut
- 211. Check Shear stress induced in Nut threads<Permissible Hence nut threads are safe against shear failure
- 212. Check Shear stress induced in Screw threads<Permissible Hence screw threads are safe against shear failure
- 213. Design of Handle
- 214. Length of handle
- 215. Design of I-Section
- 216. Permissible tensile stress along Y-Y
- 217. Maximum tensile stress along Y-Y
- 218. Maximum tensile stress along Y-Y
- 219. Ex.4.12 The following data refers to C-clamp; • Maximum force exerted by C-Clamp=4kN • Nominal diameter=12mm • Pitch =2mm, • Nut height=25mm • Type of screw=single start square thread • Coefficient of the screw friction=0.12 • Coefficient of the collar friction=0.25 • Mean collar radius=6mm • Distance between the axis of the handle and nut surface, in clamped condition=150mm • Force applied by an operator=80N • Distance between the axis of the screw and centroidal axis of vertical column of the C-clamp body=100mm
- 220. Determine; i) The length of handle, if additional length provided for gripping is 50mm; ii) The maximum shear stress in the screw body and its location; iii) The bearing pressure on the threads
- 221. Given
- 222. Design of Screw body
- 223. To find core diameter dc of screw
- 224. To find Lead angle
- 225. To find friction angle
- 226. Torque required to overcome thread friction
- 227. Torque required to overcome the collar friction
- 229. Torsional shear stress in body
- 230. Bending stress in body
- 231. Maximum Shear stress
- 232. Height of Nut
- 233. Bearing Pressure
- 234. Length of Handle
- 235. Length of handle 175
- 236. Differential screw
- 237. Differential screw It consists of two screws in series having same Hands, arranged such that the resultant motion is the difference of individual motions of the two screws. Differential screw, shown in fig. consist of lower screw with pitch p1(LH) and the upper screw with pitch p2(LH) When the nut is turned through one revolution in clockwise direction viewed from top, the top screw advances by a distance (p1-p2) in
- 238. Compound screw It consists of two screws in series having opposite hands, arranged such that the resultant motion is the sum of individual motions of the two screws. compound screw, shown in fig. consist of lower screw with pitch p1(LH) and the upper screw with pitch p2(LH) When the nut is turned through one revolution in clockwise direction viewed from top, the top screw advances by a distance (p1+p2) in
- 240. Recirculating Ball screw • In a power screw, if the sliding friction at the threads is replaced by rolling friction efficiency of screw can be improved substantially. This is achieved by screw known as recirculating ball screw. • Typical recirculating ball screw shown in fig consist of three components i.e. Screw, nut and steel balls • A screw and a nut have a semi-circular thread profile. the contact between the screw and nut threads is through the steel balls. • As the nut or screw rotates, rolling balls move along the circular grooved helical path.
- 241. Advantages of Recirculating Ball screw • In recirculating ball screw, as the sliding friction is replaced by rolling friction, efficiency is very high. • As the nut or screw is preloaded in one direction to reduce the backlash, high positional accuracy is obtained. • Because of low coefficient of friction, conversion of rotary to linear motion can be reversible. • As the nut or screw rotates, rolling balls move along the circular grooved helical path.
- 242. Applications of Recirculating Ball screw • Ball screws are used in aircraft and missiles to move control surfaces, especially for electric fly by wire. • Used in automobile power steering to translate rotary motion from an electric motor to axial motion of the steering rack. • They are also used in machine tools, robots and precision assembly equipment. • High precision ball screws are used in steppers for semiconductor manufacturing. • They are also incorporated into the actuator mechanisms of computer controlled self-pleasure devices.
- 243. Theory Questions for Practice i. Explain different types of threads used for power screws. Give advantages and limitations of each type. ii. Derive an equation for the efficiency of square threaded screw. iii. Show that efficiency of self-locking square threaded power screw is less than 50% iv. Explain with neat sketch differential screw v. Explain with neat sketch re-circulating ball screw
- 244. Ex.01 The following data refers to C-clamp; • Maximum clamping force =4000N • Nominal diameter=12mm • Pitch =2mm, • Nut height=25mm • Type of screw=single start trapezoidal thread • Coefficient of the screw friction=0.12 • Coefficient of the collar friction=0.25 • Mean collar diameter=12mm • Distance between the axis of the handle and nut surface, in clamped condition=150mm • Operator Force at the end of handle=80N Numerical for Practice
- 245. Determine; i) The length of handle, if 50mm additional length for gripping ; ii) stresses in the screw body at two critical sections; iii) The bearing pressure on the screw threads
- 246. Ex.02 A C-clamp as shown in fig. below is used on the shop floor has single-start square thread of 22mm nominal diameter and 5mm pitch. The coefficient of friction at the threads and the collar is 0.15. the mean radius of friction collar is 15mm. The capacity of the clamp is 750N. The handle is made of steel 30C8 (Syt=400MPa) it can be assumed that the operator exerts force of 20N on the handle. i. Evaluate the torque required to tighten the clamp to its full capacity. ii. Determine the length and diameter of the handle such that it will bend with a permanent set when the rated capacity of the clamp is exceeded.
- 247. Ex.03 A power screw having double start square threads nominal diameter 25mm and pitch 5mm subjected to axial load of 1000N. The outer and inner diameter of the screw collar is 50 and 20mm respectively. The coefficient of friction for collar thread and screw thread are 0.15 & 0.20 respectively. The screw rotates at 12rpm. Assume uniform wear condition, and allowable bearing pressure is 5.77N/mm2.determine, i) Power required to rotate the screw. ii) Stresses in screw body and threads iii) No. of threads of nut in engage with screw. Ex.04 A load of 600kN is to be raised and lowered by means of two square threaded screws. If the coefficient of friction between the screw and nut is 0.048, determine the size of screw and nut. Take ,P=15MPa,pitch=10mm. Find also the torque required to raise and lower the load. Ex.05 The lead screw of lathe has single start ISO metric trapezoidal threads of 52mm nominal diameter and 8mm pitch. The screw is required to exert an axial force of 2kN in order to drive the tool carriage during turning operation. The thrust is carried on a collar of 100mm outer diameter and 60mm inner diameter. The value of coefficient of friction at the screw threads and the collar are 0.15 and 0.12 respectively. The lead screw rotates at 30r.p.m evaluate i) The power required to drive the lead screw.
- 248. Ex.06 A nut and screw combination having double start square threads nominal diameter 25mm and pitch 5mm subjected to axial load of 1000N. The outer and inner diameter of the screw collar is 50mm and 20mm respectively. The coefficient of friction for collar thread and screw thread are 0.15 and 0.2 respectively. The screw rotates at 12rpm. Assume uniform wear condition and allowable bearing pressure is 5.77N/mm2 determine i) Power required to rotate the screw. ii) Stresses in screw body and threads iii) no. of threads of nut in engage with screw. Ex.07 a triple threaded power screw used in screw jack has nominal diameter of 50mm and pitch of 8mm. The threads are square and length of nut 48mm. The screw jack is used to lift load of 8kN. The coefficient of friction at the threads is 0.12. calculate i) The principal shear stress in the screw body ii) The transverse shear stresses in the screw and nut iii) The unit bearing pressure. State the condition of screw with statement.