lab 1-rocker/slider crank

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Mechanisms and Multibody Systems MECH3422 Lab One Analysis and Design of Planar Mechanisms Group Members Benjamin Martis 20365448 Jarrod Hofmann 20365617 Justin Loong Chen Ng 20984628 Christopher Yih-Ray Tay 20979725 Yua yang Yem 20977141 Stanley ChunWee Eng 20982213 REPORT PART A Aim: The aim of part A of the laboratory was to construct and analyse a quick-return slider-crank mechanism. Procedure: Step1: A mechanism was constructed from provided Lego pieces that had lengths that were in proportion to the following lengths specified in the lab hand out of r2=3; r3=9 Lego horizontal units; e=9 Lego vertical units. The mechanism was built to satisfy the optimal design specifications of the return stroke moving faster than the working stroke. Step 2: After a seemingly satisfactory mechanism was constructed using the existing mechanism as a reference, testing was needed to determine that the advance to return time ratio was greater than one. This confirmed that the mechanism constructed was indeed a quick return mechanism. Step 3: Once testing was completed the crank angle (alpha) and the stroke length were measured using a ruler, protractor and the RCX interface. Step 4: Using trigonometry the alpha and beta angles and the stroke length were calculated.

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Page 1: Lab 1-Rocker/slider crank

Mechanisms and Multibody Systems MECH3422Lab One

Analysis and Design of Planar Mechanisms

Group Members

Benjamin Martis 20365448Jarrod Hofmann 20365617Justin Loong Chen Ng 20984628Christopher Yih-Ray Tay 20979725Yua yang Yem 20977141Stanley ChunWee Eng 20982213

REPORT PART A

Aim:The aim of part A of the laboratory was to construct and analyse a quick-return slider-crank mechanism.

Procedure:Step1: A mechanism was constructed from provided Lego pieces that had lengths that were in proportion to the following lengths specified in the lab hand out of r2=3; r3=9 Lego horizontal units; e=9 Lego vertical units. The mechanism was built to satisfy the optimal design specifications of the return stroke moving faster than the working stroke.

Step 2: After a seemingly satisfactory mechanism was constructed using the existing mechanism as a reference, testing was needed to determine that the advance to return time ratio was greater than one. This confirmed that the mechanism constructed was indeed a quick return mechanism.

Step 3: Once testing was completed the crank angle (alpha) and the stroke length were measured using a ruler, protractor and the RCX interface.

Step 4: Using trigonometry the alpha and beta angles and the stroke length were calculated.

Page 2: Lab 1-Rocker/slider crank

Diagram of Quick-Return Slider-Crank Mechanism:

Fig 1: Overview

Fig 2: Front view

Page 3: Lab 1-Rocker/slider crank

Fig 3: Top view

REPORT PART B

Aim:The aim of this section was to construct and analyse a crank-and-rocker mechanism.

Procedure:Step 1: First we calculated alpha from a given advance-to-return time ratio.

Step 2: A graph was then constructed that satisfied all the below specifications. alpha = 24.8 deg (calculated in Step 1);output angle (Ф) = 75 deg;rocker length r4=6[horizontal Lego units]

Step 3: A mechanism was then constructed from the dimensions determined from the graph. (Rounding to the nearest Lego unit was necessary).

Step 4: The mechanism was then tested to see if it complied sufficiently to the design specifications. An RCX interface reading was obtained from the constructed mechanism and the alpha and Q values were determined.

Step 5: As our alpha reading was deemed satisfactory, no redesign was required.

Step 6: Using trigonometry the alpha and rocker angles were calculated.

Page 4: Lab 1-Rocker/slider crank

Diagram of Crank-Rocker Mechanism:

Fig 4: Overview

Fig 5: Side view

Page 5: Lab 1-Rocker/slider crank

Fig 6: Side view from back

Fig 7: Top view

Page 6: Lab 1-Rocker/slider crank

RESULTS

Part A

Trigonometry (No Ruler or Protractor, Count the Lego Units)

r2 = 24 mm Stroke = 53.18 mm

r3 = 72 mm α = 199.4 ° β = 160.6 °

e = 28.8 mm Q = 1.24 __

RCX Interface

Reading : 221 x 0.9 = 198.9 = α (°)

Q = α / (360 – α) = 1.23_

Measurement (Use Ruler and Protractor)

Ruler: Stroke = 53 mm

Protractor: α = 200 ° Q = α / (360 – α ) = 1.25__

Part B

Trigonometry (AS-Built dimensions, No Ruler or Protractor, Count the Lego Units)

r2 = 24 mm

r3 = 72 mm α = 20.9 °

r4 = 48 mm Q = 1.26 __

rhorizontal = 64 mm Rocker angle, ɸ = 68.4 °

rvertical = 9.6 mm

RCX Interface

Reading : 217 x 0.9 = 195.3 = α + 180 (°)

α = 15.3 ° Q = (180 + α) / (180 – α) = 1.19_

Measurement (Use Ruler and Protractor)

Protractor: Rocker angle, ɸ = 74 °

Protractor: α = 21 ° Q = (180 + α) / (180 – α) = 1.26_

Calculate

Calculate

Page 7: Lab 1-Rocker/slider crank
Page 8: Lab 1-Rocker/slider crank
Page 9: Lab 1-Rocker/slider crank

SOURCES OF ERROR WHICH REQUIRE IMPROVEMENT:

The set-up and procedure of this experiment meant there were discrepancies between the measured and calculated values which we obtained. Possible methods of improving the accuracy of this experiment are addressed below.

There were discrepancies between which Lego pieces were chosen for the built design, with the choice of using either Technics beam blocks or shafts with end attachments. A more accurate means of constructing the device would give better results, so a more flexible arrangement of building materials allowing finer adjustment would be an improvement.

In addition the fixed size of horizontal and vertical lego units meant that rounding of the calculated value of alpha was required for part B. This induced error into the mechanism. Having pieces of lego with different pitched bores would help eliminate this problem.

A mechanical method to start and stop the mechanism when measuring the angle of advance would eliminate human judgement error.

Depending on how the mechanism was constructed with the Lego pieces the mechanism can contain extra degrees of freedom. These unnecessary degrees of freedom allow slight rotations and an overall skewness in the mechanism. By making the mechanism more rigid the motion will lie in a 2D plane as it should which could slightly improve the accuracy.

The motion of the quick-return slider-crank mechanism was slightly jerky due to the friction in the Lego pieces. Eliminating this with lubrication will allow a smoother rotation and more accurate angle measurement in terms of the starting and stopping position.

It was very difficult to measure the angle on the mechanism with a protractor. Another means of measuring would be preferred, for example a different shaped protractor could be used, a profile photo of the mechanism could be taken and the angle measured on that, or an adjustable triangle could be slotted into the mechanism to gain the shape of the angle, and then traced onto paper.

FINAL CONCLUSION

Both of our mechanisms were reasonably accurate, as the three different Q values we obtained by measuring, calculating and testing with the RCX interface, were very close for both mechanisms. For part A, we calculated 1.24 using trigonometry, the RCX interface gave a reading of 1.23 and our physical measurements produced 1.25. For part B the values were also close, with the calculated value being 1.26, the RCX interface giving 1.19 and the physical measurements 1.26.