simple screw jack

Objective: To Calculate the efficiency of simple screw jack.

Introduction

Machine: It is a device which is use for doing a particular work from receives energy in some available form.

Lifting Machine: It is a device which is use to overcome a force or load (W) applied at one point by mean of another force called effort (P).

Types of Machine:

1. Simple machine
2. Compound machine

• · Simple Machine: Have only one point for the application of effort and one point for load.eg: Lever, screw jack etc.
• · Compound Machine: has more than one point for the application of effort and for load. eg: Printing machine, milling machine, planer, shaper etc

IMPORTANT TERMS

• Mechanical advantage of a machine (M.A.): It is the ratio of the weight lifted (W) to the effort applied (P).
• M.A. = W/P

• Velocity ratio (V.R.): It is the ratio of the distance (y) moved by the effort to the distance (x) moved by the load.
• V.R = y/x

• Input of a machine: It is the work done on the machine. In a lifting machine, it is measured by the product of effort and the distance through which it has moved (i.e., P.y).

• Output of a machine: It is the actual work done by the machine. In a lifting machine it is measured by the product of the weight lifted and the distance through which it has been lifted i.e., (W.x).

• Efficiency of a machine (η): It is the ratio of output to the input of a machine.
• η = Output/Input

• Ideal machine: A machine is said to be ideal if its efficiency is 100%. In this case, output is equal to input.

Whereas:

W= Load lifted by the machine;

P = Effort required to lift the load;

y = Distance moved by the effort, in lifting the load;

x = Distance moved by the load;

η = Efficiency of the machine.

SIMPLE SCREW JACK

It is a device employed for lifting heavy loads which are usually centrally loaded upon it. Horizontal power is applied with the lever (or handle).

Formula Used

Let L = Length of lever (or power arm)

P = The effort applied

W= The load lifted

p = Pitch of the screw

Suppose, screw has taken one full revolution,

Distance moved by the load = p

Distance moved by the effort = 2πL

V.R. = (Distance moved by P) / (Distance moved by W) = 2πL / p

If the screw is double threaded then for one revolution of power arm the load will be lifted up through twice the pitch.

Hence, V.R. for double threaded screw, V.R = 2πL / 2p = πL / p

M.A = W / P

η = M.A / V.R

Procedure:

1. Firstly stabilize the simple screw jack machine and wrap the cord around the load drum and pass it over the pulley.
2. Put some weight on the load drum. And add the some effort to the effort hanger on the pulleys.
3. Hit the machine with some material, thus you will see some kind of movement in the load drum.
4. Write down the initial reading in the observation table.
5. After taking the initial reading we put the some load on the load drum.
6. After this just increase the effort on the effort pulleys (either to the left or to the right)
7. Again hit the machine with some material, thus you will see another movement in the load drum.
8. Write down the second reading in the observation table.
9. After this apply the above procedure, four to five times with gradually increasing the Load as well as Effort to the load drum and effort pulley respectively.
10. Write down the all reading in the given observation table.
11. Measure the radius of the load drum and pitch of screw.
12. Calculate the MA, VR and η of machine.

Observation table:

S.No.	Load (W) in gram.	Effort (P) in gram.
PA	PB	P = PA + PB
1.
2.
3.
4.
5.

Diagram:

Precautions:

1. Lubricate the screw before starting the experiment.
2. Trapping should be done after adding the weight in the effort hanger.
3. Overlapping of string should not be there.

Source of error:

1. Frictions in the pulley.
2. Effort being pulled suddenly.

Result: The efficiency of the simple screw jack is……………………………..