Lab report

Discussion a-Whitworth Quick-Return Mechanism: The Whitworth quick return mechanism as noticed during the experiment converts the rotary motion which is the input into reciprocating motion. The Whitworth mechanism has one degree of freedom, because it requires only one variable to completely define the output produced by the driver which is linear reciprocating motion. The movement of the output slider as shown in figure 7 starts with a forward motion of the slider this is because at the beginning at an angle of 0 o the slider was at a point with in the forward stroke it reaches the maximum distance for the fixed centre of the crank at about 50 o this is shown by change in direction of the slop of the curve then the backwards stroke starts which continues for the next 110 o until the slider reaches the minimum distance form the fixed centre of the crank at about 160 o this is shown by change in the direction of the curve. After this the forward stroke starts which continues till end of the revolution at 360 o. It can be seen from figure 7 that the slider displaces with a greater magnitude during backward stroke then the forward stroke as a result it takes less time to return backwards then it takes to complete the forward stroke. This result is also proved by the time ratio which is theoretically and experimentally found to be 2.25 and 2.27 respectively. Since the time ratio is the ratio between time taken for forward stroke to the time taken for backward stroke so a value more than 1 means that it takes less time for the backward stroke, therefore it is termed as a quick return mechanism. Reasons for errors can be as follows: 

Parallax error made during acquisition of data.



The accuracy of the input is 0.5 o while for the output it is 0.5mm.



The mechanism is made of moving parts connected together by joints which have become loser due to extensive usage therefore may not transmit force effectively.



The slider movement is effected by variable friction of the base over which it slides because the surface may have become rougher at certain areas or may have defects.

b-Four-Bar Linkage Mechanism:

In the experiment the crank link(driver) makes a complete one revolution while the Driven link(rocker) oscillates a certain angle. Therefore this four-bar Linkage Mechanism is called Crank-rocker mechanism. It is also noted that the Driven link return back to its initial position at the end of the revolution. Two cases are as follows:

Case 1: Keeping length of Coupler maximum and driven link length minimum. The figure 9 shows the starting output angle at 152 o the angle decreases with increasing output until it reaches the value of 98 o then the output angle starts increasing this indicates change in rotation axis so this value can be considered as the extreme value for the rocker. The output angle increases until crank completes one revolution and final value of the output angle is 152 o. Case 2: Keeping length of Coupler minimum and driven link length maximum. The figure 9 shows the starting output angle at 189 o the angle decreases with increasing output until it reaches the value of 158 o then the output angle starts increasing this indicates change in rotation axis so this value can be considered as the extreme value for the rocker. The output angle increases until crank completes one revolution and final value of the output angle is 189 o. In general the figure 9 shows that change in output angle for case 1 is much more than change in output angle for case 2 this is because of the difference in coupler and driven links length. So keeping variation in input angles same for both cases is can be concluded that a longer coupler and a shorter driven link would result in higher variation in output angle as compared to a shorter coupler and a longer driven link. The same is also true for velocity as shown in figure 10 that the which shows that the variation in velocity in case 1 is much more than in case 2. It should also be noted that the variation in lengths of coupler and driven link was kept with in limits so that the mechanism still remains a Grashof mechanism as well a Crank rocker mechanism as following. For the mechanism to be Grashof mechanism rmax + rmin˂ ra + rb Case 1: 220 + 50 ˂ 200 + 100

250 ˂ 300 therefore Grashof mechanism Case 2: 200 + 50 ˂ 200 +160 250 ˂ 360 therefore Grashof To be Crank rocker rmin = Crank link or driver link In both case 1 and 2 Crank link is the smallest link there for it is a Crank rocker mechanism.

Reasons for error can be as follows: 

Parallax error made during acquisition of data.



The accuracy of the mechanism is 0.5 o.



The mechanism is made of moving parts connected together by joints which have become loser due to extensive usage therefore may not transmit force effectively.

c-Geneva-Stop Mechanism As the input angle increases the output angle increases with it until 145 o then the output angle remains constant for any increase to the input as shown in figure 11. The output velocity shown in figure 12 decreases with increasing input angle until 145 o. This shows the driver will have to make more the one revolution in order for the driven to complete one revolution. This type of motion is called intermittent rotary motion. Also the engaging angle is 90 o while the disengaging angle is 270 o. Reasons for errors are as follows: 

Parallax error made during acquisition of data.



The accuracy of the mechanism is 0.5 o.

d-Slider-Crank Mechanism:

During the experiment the crank is given a rotational input which is transmitted to the slider which producing a linear output. The slider crank has a degree of freedom of one since only one variable completely describes the motion of the slider. As shown in figure 13 at an angle of 270 o at the minimum distance from the fixed centre of the driver as the input angle is increased the displacement increases and the slider slides forward until at 90

o

it reaches maximum distance

from the fixed centre of crank. After which the slider slides backwards with increasing angle until at 270 o the slider reaches the starting position. It can be shown from figure 13 and 14 that the experimental profile of the plot is very near to the theoretical profile of the plot. Reasons for errors can be as follows: 

Parallax error made during acquisition of data.



The accuracy of the input is 0.5 o while for the output it is 0.5mm.



The mechanism is made of moving parts connected together by joints which have become loser due to extensive usage therefore may not transmit force effectively.



The slider movement is effected by variable friction of the base over which it slides because the surface may have become rougher at certain areas or may have defects.

Conclusion a-Whitworth Quick-Return Mechanism: The experiment proves that the Whitworth mechanism is a quick returning mechanism with a time ratio of 2.27 which is greater that 1. So backward or return time for the slider will be less than forward time of the slider from the time ratio value it can assumed that forward stroke will approximately require double the time as compared to backward stroke.

b-Four-bar linkage Mechanism: The four-bar linkage forms a crank rocker mechanism where the driver make a complete revolution while the driven oscillates. So it converts rotational motion into oscillatory motion. From the experiment it can be concluded that the magnitude of variation in angles of the driven link is affected by the lengths of both the coupler and driven links.

c-Geneva-Stop Mechanism: The input for the Geneva-Stop mechanism is the rotational motion of the crank while the output is the intermittent rotary motion.

d-Slider-Crank Mechanism: The input for Slider-Crank mechanism experiment is rotational while the output is linear motion of the slider. Therefore the slider crank mechanism can be used to transmit motion while converting the rotational motion input to linear motion output or vise versa.