Objective: To Calculate the efficiency of simple screw jack.
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:
- Simple machine
- 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
- 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.
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).
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
- Firstly stabilize the simple screw jack machine and wrap the cord around the load drum and pass it over the pulley.
- Put some weight on the load drum. And add the some effort to the effort hanger on the pulleys.
- Hit the machine with some material, thus you will see some kind of movement in the load drum.
- Write down the initial reading in the observation table.
- After taking the initial reading we put the some load on the load drum.
- After this just increase the effort on the effort pulleys (either to the left or to the right)
- Again hit the machine with some material, thus you will see another movement in the load drum.
- Write down the second reading in the observation table.
- 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.
- Write down the all reading in the given observation table.
- Measure the radius of the load drum and pitch of screw.
- Calculate the MA, VR and η of machine.
Load (W) in gram.
Effort (P) in gram.
P = PA + PB
- Lubricate the screw before starting the experiment.
- Trapping should be done after adding the weight in the effort hanger.
- Overlapping of string should not be there.
Source of error:
- Frictions in the pulley.
- Effort being pulled suddenly.
Result: The efficiency of the simple screw jack is……………………………..