2. Simple Lifting Machines
Chapter 11
Simple Lifting Machines

3. Learning Objectives Introduction Types of Lifting Machines
Simple Wheel and Axle
Differential Wheel and Axle
Weston’s Differential Pulley Block
Geared Pulley Block
Worm and Worm Wheel
Worm Geared Pulley Block
Single Purchase Crab Winch
Double Purchase Crab Winch
Simple Pulley
First System of Pulleys
Second System of Pulleys
Third System of Pulleys
Simple Screw Jack
Differential Screw Jack
Worm Geared Screw Jack

4. Types of Lifting Machines
Introduction
In the last chapter, we have discussed the principles of lifting machines. Now in this chapter, we shall discuss the applications of these principles on a few lifting machines.
Types of Lifting Machines
These days, there are many types of lifting machines which are available in the market. But the basic principle, on which all these machines are based, is the same. It will be interesting to know that engineers who have designed (or invented) these machines have tried to increase the velocity ratio of their respective lifting machines. A little consideration will show, that if the efficiency of a machine remains almost the same, then increase in the velocity ratio must increase its mechanical advantage. The increased mechanical advantage, of a machine, means the application of a smaller force to lift the same load; or to lift a heavier load with the application of the same force.

5. The following lifting machines, which are important from the subject point of view, will be discussed in the following pages:
Simple wheel and axle.
Differential wheel and axle.
Weston’s differential pulley block.
Geared pulley block.
Worm and worm wheel.
Worm geared pulley block.
Single purchase crab winch.
Double purchase crab winch.
Pulleys:
(a) First system of pulleys.
(b) Second system of pulleys.
(c) Third system of pulleys.
Simple screw jack.
Differential screw jack.
Worm geared screw jack.

6. Simple Wheel and Axle
Fig Simple wheel and axle.
In Fig is shown a simple wheel and axle, in which the wheel A and axle B are keyed to the same shaft. The shaft is mounted on ball bearings, order to reduce the frictional resistance to a minimum. A string is wound round the axle B, which carries the load to be lifted. A second string is wound round the wheel A in the opposite direction to that of the string on B. Let D = Diameter of effort wheel, d = Diameter of the load axle, W = Load lifted, and P = Effort applied to lift the load. One end of the string is fixed to the wheel, while the other is free and the effort is applied to this end. Since the two strings are wound in opposite directions, therefore a downward motion of the effort (P) will raise the load (W).

7. Since the wheel as well as the axle are keyed to the same shaft, therefore when the wheel rotates through one revolution, the axle will also rotate through one revolution. We know that displacement of the effort in one revolution of effort wheel A, = pD ...(i) and displacement of the load in one revolution = pd ...(ii)

8. Example
A drum weighing 60 N and holding 420N of water is to be raised from a well by means of wheel and axle. The axle is 100 mm diameter and the wheel is 500 mm diameter. If a force of 120 N has to be applied to the wheel, find (i) mechanical advantage, (ii) velocity ratio and (iii) efficiency of the machine.
Solution
Given: Total load to be lifted (W) = = 480 N; Diameter of the load axle (d) = 100 mm; Diameter of effort wheel (D) = 500 mm and effort (P) = 120 N.
Mechanical advantage
We know that mechanical advantage

9. Differential Wheel and Axle
Fig Differential wheel and axle.
It is an improved form of simple wheel and axle, in which the velocity ratio is intensified with the help of load axle. In fig is shown a differential wheel and axle. In this case, the load axle BC is made up of two parts of different diameters. Like simple wheel and axle, the wheel A, and the axles B and C are keyed to the same shaft, which is mounted on ball bearings in order to reduce the frictional resistance to a minimum. The effort string is wound round the wheel A. Another string wound round the axle B, which after passing round the pulley (to which the weigt W is attached) is wound round the axle C in opposite direction to that of the axle B; care being taken to wind the string on the wheel A and axle C in the same direction. As a result of this, when the string unwinds from the wheel A, the other string also unwinds from the axle C. But it winds on the axle B as shown in Fig

10. Let D =Diameter of the effort wheel A, d1=Diameter of the axle B, d2=Diameter fo the axle C, W =Weight lifted by the machine, and P =Effort applied to lift the weight. We know that displacement of the effort in one revolution of effort wheel A = pD ...(i) \ Length of string, which will wound on axle B in one revolution = pd1 and length of string, which will unwound from axle C in one revolution = pd2 \ Length of string which will wound in one revolution = p d1 – p d2 = p (d1 – d2)

11. Example
In a differential wheel and axle, the diameter of the effort wheel is 400 mm. The radii of the axles are 150 mm and 100 mm respectively. The diameter of the rope is 10 mm.
Find the load which can be lifted by an effort of 25 N assuming the efficiency of the machine to be 84%.
Solution
Given: Diameter of effort wheel = 480 mm; Radii of axles = 150 mm and 100 mm or diameter of axles = 300 mm and 200 mm; Diameter of rope = 10 mm; Effort (P) = 25 N and efficiency (h) = 84% = 0.84.
Let W = Load that can be lifted by the machine. We know that effective diameter of the effort wheel,
D = = 410 mm
 and effective diameters of axles,
  d1 = = 310 mm and d2 = = 210 mm
We also know that velocity ratio of a differential wheel and axle,

12. Weston’s Differential Pulley Block.
It consists of two blocks A and B. The upper block A has two pulley (P1 and P2) one having its diameter a little larger than that of the other. The Pulleys turn together as one pulley, i.e., both of them behave as one pulley with two grooves. The lower block B also carries a pulley, to which the load W is attached. An endless (i.e., a continuous) chain passes round the pulleys then round the lower block pulley and finally round the pulley P2 (i.e., smaller of the upper pulleys). The remaining chain hangs slack and is joined to the first portion of the chain as shown in Fig
Fig Differential pulley block.

13. The effort P is applied to the chain passing over the pulley P1 (i. e
The effort P is applied to the chain passing over the pulley P1 (i.e., larger of upper pulleys) as shown in Fig To prevent the chain from slipping, projection are provided in the grooves of both the upper pulleys. D= Diameter of the pulley P2, d = Diameter of the pulley P2, W = Weight lifted, and P = Effort applied to lift the weight. We know that displacement of the effort in one revolution of the upper pulley block, = pD ...(i) This is also equal to the length of the chain pulled over the larger pulley. Since the smaller pulley also turns with the larger one, therefore length of the chain released by the smaller pulley = pd \ Net shortening of the chain = pD – pd = p (D – d) This shortening of chain will be equally divided between the two portions of the chain, supporting the load. Therefore distance through which the load will move up

14. Example
In a differential pulley block, a load of 1800 N is raised by an effort of 100 N. The number of teeth on the larger and smaller blocks are 12 and 11 respectively. Find the velocity ratio, mechanical advantage and efficiency of the machine.
Solution
Given: Load (W) = 1800 N; Effort applied (P) = 180 N; Number of teeth on the larger block (T1) = 12 and number of keeth on the smaller block (T2) = 11.
Velocity ratio
We know that velocity ratio

15. Geared Pulley Block.
It is an improved form of a differential pulley block in which the velocity ratio is intensified with the help of gears. A geared pulley block consists of a cogwheel A, around which is passed an endless chain. A smaller gear wheel B, known as pinion, is keyed to the same shaft as that of A.
The wheel axle B is geared with another bigger wheel C, called the spur wheel. A cogwheel D is keyed to the same shaft as that of spur wheel C.
The load is attached to a chain, that passes over the cogwheel D. The effort is applied to the endless chain, which passes over the wheel A as shown in Fig
Fig Geared pulley block.

16. and no. of revolution made by the pinion B
Let T1= No. of cogs on wheel A(known as effort wheel),
  T2= No. of teeth on wheel B(known as pinion),
 T3= No. of teeth on the wheel C. known as spur wheel, and
  T4= No. of cogs on the wheel D known as load wheel.
We know that distance moved by the effort in one revolution of the cogwheel A,
 = T1
and no. of revolution made by the pinion B 1
No. of revolutions made by the spur wheel C  

17. Example
A geared pulley block, used to lift a load, has the following dimensions: No. of cogs on the effort wheel = 90, No. of cogs on the load wheel = 8 No. of teeth on the pinion = 25, No. of teeth on the spur wheel = 40 Find the maximum load that can be lifted by an effort of 50 N on machine. Take efficiency of the pulley block as 75%.
Solution
Given: No. of cogs on the effort wheel (T1) = 90; No. of cogs on the pinion (T2) = 25; No. of teeth on the spur wheel (T3) = 40; No. of teeth on the load wheel (T4) = 8; Effort (P) = 50 N and efficiency (h) = 75% = 0.75.
 Let W = Load that can be lifted by the machine.
 We know that velocity ratio of a geared pulley block,

18. Worm and Worm Wheel
Fig Worm and worm wheel
It consists of a square threaded screw, S (known as worm) and a toothed wheel (known as worm wheel) geared with each other, as shown in Fig A wheel A is attached to the worm, over which passes a rope as shown in the figure. Sometimes, a handle is also fixed to the worm (instead of the wheel). A load drum is securely mounted on the worm wheel. Let D = Diameter of the effort wheel, r = Radius of the load drum W = Load lifted, P = Effort applied to lift the load, and T = No. of teeth on the worm wheel. We know that distance moved by the effort in one revolution of the wheel (or handle) = pD ...(i)

19. If the worm is single-threaded (i. e
If the worm is single-threaded (i.e., for one revolution of the wheel A, the screw S pushes the worm wheel through one teeth), then the load drum will move through
Example
In a double threaded worm and worm wheel, the number of teeth on the worm wheel is 60. The diameter of the effort wheel is 250 mm and that of the load drum is 100 mm. Calculate the velocity ratio. If the efficiency of the machine is 50%, determine the effort required to lift a load of 300 N.

20. Solution
Given : No. of threads (n) = 2; No. of teeth on the worm wheel (T) = 60; Diameter of effort wheel = 250 mm; Diameter of load drum = 100 mm or radius (r) = 50 mm; Efficiency (h) = 50% = 0.5 and load to be lifted (W) = 300 N. Velocity ratio of the machine We know that velocity ratio of a worm and worm wheel, Effort required to lift the load Let P = Effort required to lift the load. We also know that mechanical advantage,

21. Worm Geared Pulley Block
Fig Worm geared pulley block.
It is an improved form of worm and worm wheel, in which the velocity ratio is intensified with the help of a pulley block. It consists of a square threaded screw S and a toothed wheel. One end of the string is fastened to the frame. This string passes round the load drum B and then over the pulley A (which is keyed to the wheel). The other end of the string is also fixed to the frame. The effort is applied to the wheel C by a rope as shown in Fig Let D = Diameter of effort wheel, r = Radius of pulley W = Load lifted, P = Effort applied to lift the load, and T = No. of teeth on the worm wheel.

22. We know that distance moved by effort in one revolution of wheel = pD
If the worm is single threaded (i.e., for one revolution of wheel C, the screw S pushes the worm wheel through one tooth) then the worm wheel and pulley A will move through.

23. Example
A worm geared pulley block has its effort wheel of 200 mm diameter. The worm is single threaded and the worm wheel has 60 teeth. The load drum is of 80 mm diameter. Find the efficiency of the block, if an effort of 75 N is required to lift a load of 9 kN.
Solution
Given: Diameter of effort wheel = 200 mm; No. of teeth in worm wheel (T) = 60; Diameter of load drum = 80 mm or radius (r) = 40 mm; Load to be lifted (W) = 9 kN = 9000 N and effort (P) = 75 N.
 We know that velocity ratio of a worm geared pulley block,

24. Single Purchase Crab Winch
Fig Single purchase crab winch
In single purchase crab winch, a rope is fixed to the drum and is wound a few turns round it. The free end of the rope carries the load W. A toothed wheel A is rigidly mounted on the load drum. Another toothed wheel B, called pinion, is geared with the toothed wheel A as shown in Fig The effort is applied at the end of the handle to rotate it. Let T1= No. of teeth on the main gear (or spur wheel) A, T2= No. of teeth on the pinion B, l= Length of the handle, r= Radius of the load drum. W= Load lifted, and P = Effort applied to lift the load.

25. We know that distance moved by the effort in one revolution of the handle, = 2pl ...(i) \ No. of revolutions made by the pinion B = 1 and no. of revolutions made by the wheel A T2 = T1

26. Example
A single purchase crab winch, has the following details: Length of lever = 700 mm Number of pinion teeth = 12 Number of spur gear teeth = 96 Diameter of load axle = 200 mm It is observed that an effort of 60 N can lift a load of 1800 N and an effort of 120 N can lift a load of 3960 N. What is the law of the machine ? Also find efficiency of the machine in both the cases.
Solution
Given: Length of lever (l) = 700 mm; No. of pinion teeth (T2) = 12; No. of spur geer teeth (T1) = 96 and dia of load axle = 200 mm or radius (r) = 200/2 = 100 mm.
 (i) Law of the machine
When P1 = 60 N, W1 = 1800 N and when P2 = 120 N, W2 = 3960 N. Substituting the values of P and W in the law of the machine i.e., P = mW + C
  = (m × 1800) + C (i)
 and = (m × 3960) + C (ii)
 Subtracting equation (i) from equation (ii)
  = m × 2160