D V RAMANAREDDY
SIMPLE SCREW JACK
AIM
• To calculate mechanical advantage, velocity ratio, efficiency and to obtain the law of machine for Screw Jack.
• To study Gear mechanism of Simple Screw Jack, velocity Ratio, Mechanical Advantage, Efficiency.
THEORY
Machine
Any lifting machine has the principle of overcoming a greater force called Load by means of comparatively a smaller force called effort.
Velocity Ratio (VR)
It is the ratio of displacement of effort to the corresponding displacement of load
Displacement of Effort (y) Velocity Ratio = -------------------------------
Displacement of Load (x)
Mechanical Advantage (MA)
The Mechanical Advantage (MA) occurring from arrangement of different parts of lifting machine is expressed as the ratio of load lifted and the corresponding effort required to lift it.
Load lifted (W)
Mechanical Advantage (MA) = ---------------------
Effort Applied (P)
Efficiency
It is the ratio of output to input and expressed as percentage.
Reversibility of a Machine
The property of a machine to work in reverse direction on removal of effort is called as reversibility of machine.
Simple Screw Jack.
It is a lifting machine consisting of
1. A vertical threaded spindle (S).
2. Heavy body of the screw jack with centrally drilled hole having internal threads through which spindle moves.
3. A Platform which is fixed on the top of vertical spindle.
4. A guide pulley to facilitate application of horizontal tangential effort tangential to circumference of disc.
DESCRIPTION
It consists of
1. Working model of Simple Screw Jack,
2. load hanger,
3. effort pan set of weights (20, 50, 100 & 200 gms.)
4. Load Pan Weights (2, 4 or 5 kg), measuring tape etc.
PROCEDURE
1. Observe the model of Simple Screw Jack, Identify effort wheel, Identify various components such as pitch of screw, vertical spindle, screw jack body, internal threading, loading platform, guide pulley, arrangement for applying the effort and load.
2. Set up the machine. Attach heavy load (say1Kg) and find corresponding effort (P),by gradually increasing the magnitude so that when effort moves down, load (W) just starts moving up. Mark the motions in the diagram. Then withdraw (remove) the effort (P), and observe whether the load now moves down or not and hence check the reversibility of the machine.
3. Apply the loads on the platform.
4. Apply efforts for above loads on the effort pan and increase it gradually till effort just moves downwards and load lifted up simultaneously.
5. Record the observation of load and corresponding effort in observation table.
6. Measure the circumference of Disc with the help of string and measuring tape. Measure the pitch of the screw by using vernier scale.
7. Calculate velocity ratio by the relation as given in sample calculation.
8. Compute all the columns of observation table.
OBSERVATIONS :
Table for calculation of M.A. and efficiency
|Sr. |Load |Effort |MA |Velocity |Efficiency |Ideal Effort |Effort lost in |
|No. |(W) |(P) |(M.A. = W/P) |Ratio |((=(MA/VR) |(Pi=W/VR) |friction |
| |In ‘N’ |In ‘N’ | |(V.R.) |x 100% |in ‘N’ |(Pf=P-Pi) in |
| | | | | | | |‘N’ |
|1 | | | | | | | |
|2 | | | | | | | |
|3 | | | | | | | |
|4 | | | | | | | |
|5 | | | | | | | |
SAMPLE CALCULATIONS :
For reading No……
• Mechanical Advantage = Load/Effort = W/P
M.A. = ……./……..
= ……………..
M.A.
• Efficiency = ( = -------- x 100% = ___________ X 100%
V.R.
= ……………. %
Av.Efficiency = (av = ((1 + (2 + (3 + (4 + (5)/5
= ………………………………
= …………. %
• Pi = W/VR = ……………….. = …………………
• Pf = P - Pi = ……………….. = …………………
RESULT :
The average efficiency of the machine is ………….%
As efficiency of the machine is…………..% which is …………%
( ................
................
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