CAD - Design of a Kinematic Device - Assessment Answer

December 05, 2018
Author : Sara Lanning

Solution Code: 1AJDH

Question: CAD Assignment

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CAD Assignment

Case Scenario/ Task

You are to design a kinematic device that you can later manufacture (using technology at Deakin – laser cutting,

3D printing etc.) and test.

Your design must have a central rotating part that causes periodic motion elsewhere in the device (ideally

this motion will not be symmetric).

Your design must allow for subsequent testing if it is manufactured – i.e. you must design it with space to place sensors in mind.

The deliverable for this assessment item will be a report (max. 10 pages) which covers the design of the device.

This report should also cover all of the dynamics and demonstrate that the device meets the design criteria

(manufacturable (at Deakin), testable, central rotating part and have at least 1 component that has subsequent

motion)

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Solution: CAD Assignment

1.1   Kinematics

Kinetics is a study of motion of various points, bodies and systems. This part of mechanics does not consider mass into action and neither the forces applied has any influence. The only concern for the kinematics is the motion geometry study. Wherever kinematics is considered it is there to study the initial conditions and later on deriving a suitable motion for the link. There is a lot more to do with the geometry of the model as it not only determines the position of the model at a period of time but is also responsible for determining the velocity and acceleration of the particle or bodies when initial states are provided. The other branch known as kinetics is responsible for the study of forces in the system.

Kinematics further gives birth to Kinematics analysis which is a measuring process of any unknown variables from the present state. The kinematic analysis further gives us the range of movement of a link or a body in a mechanism

Kinematic synthesis is a process to design a mechanism in a desired range of motion. This part of kinematics specializes and focusses on the algebric geometry to make the full advantage of a mechanism.

In this project a brief understanding is required to proceed further while designing a kinematic device.

1.1   Designing a kinematic device

A kinematic device is used to convert a certain motion to another required motion. Whether it is reciprocatory, angular or rotary motion, any motion can be transferred to another using a number of inversions and fixing the degree of freedom. As for an example in Oldham Coupling, the coupling is a kinematic device which is used to correct the misalignment caused in parallel connection the two rotating device is thus capable of transferring power as required inspite of their misalignment. The Quick return mechanism is another example of kinematic device which uses rotary motion to transform it into reciprocatory motion. They find their application in shaping machine where a forward and return stroke is obtained from the rotary motion of motor shaft.

Thus it can be observed that a kinematic device can be used to sort out many issues from limited options available. Any desired motion can be obtained from a fixed type of movement.

1.1   Scotch Yoke Mechanism

This mechanism uses a yoke frame consisting of opposite parallel tracks along with rollers if necessary. These rollers are represented by either a rectangular block or rod. It is used to convert rotational motion of a crank shaft to a reciprocatory motion. The yoke frame present in the parallel pair of tracks consists of parallel pair of tracks which further includes a block or piston. This is the inversion of a four bar mechanism.

1.     Design Constraints

1.1   Design Constraints

  1. The model must be small in size and can fit in an enclosure of 25 cm x 25 cm x 25 cm.
  2. The model should be easily manufactured using 3D printer.
  3. The design must have a central rotating part.
  4. The central rotating part must cause a movement at other end of the model.

1.2   Design Intent

Here in the project, the main purpose will be to make out a useful model using a similar mechanism. There are number of applications for such type of mechanism. The use of periodic / harmonic motion shall be used to form a working model.

The design must be such that it produces a periodic motion as required. So a two port valve for a Dosing Pump can be thought of. Two inlets and outlets can be controlled using a single kinematic device. The device must have two central ports opening at regular intervals. With the help of this device, two different chemicals or fluid can be mixed periodically in a solution by adjusting the speed of the rotation.

Further the periodic motion of the valve can be adjusted by adding or subtracting kinetic links.The rough sketch of the selected mechanism was drawn as below

Case 1 scotch yoke mechanism with small slit

In this case the sketch was a simple mechanism with an attached plate at the end of the parallel rod attached to the frame. The initial sketch was designed as follows. The model was not selected due to its complexity of shape. The chances of failure of valve were also higher.

Case 2 scotch yoke mechanism with thick slit

In this case the sketch was further modified by increasing the thickness of plate to resolve the issue of failure under large pressure. But the sealing issue was different and posed a serious design issue. The design was also not acceptable for its construction reason.

Case 3 scotch yoke mechanism without slit

In this case the additional plate was removed from the previous design to make it moresimple in construction and perform the job equally. The design worked as per requirement and solved the issues confronted in the previous design.

Final selection

Case 3 was finally selected for designing in the software to analyse, assemble and simulate the design further.

2.     Software modelling

The software used for the purpose is Solidworks. A number of parts were made for the purpose. The part modelling was done in the software itself.

2.1   Rotating part

This part is directly coupled with the rotating shaft of the motor on one side. The other part of the side is kept is in constrained motion with the yoke frame. It has a circular extrusion at a distance of 70 mm which is coupled with the yoke. The details of the part model have been shown with the help of a diagram.

In this project, the model design will have its base from scotch yoke motion, but not the same. But before proceeding to the design concept the mechanism should be understood carefully.

Rotating Part

3.2   Yoke frame

The yoke frame installed in the mechanism has been kept simple without use of any slits or slots. The cylindrical block or rectangular block has been replaced by the rectangular rod itself which is reciprocating to and fro. The frame has a slot of 20 mm inside where the pin attached to the rotating part is attached and move as a constrained pair.

yoke frame motion restrictor

3.3   Motion guide

This part is attached with the base plate in order to guide the frame in a line. The reciprocatory motion of the yoke frame is all due to the constrained motion of pin in the rotating part. This part has an inlet as well as an outlet on its top-down surface. These ports can be used to connect a flow of any fluid which requires dosing after a break.

motion guide

This part contains the important design of ports which can be utilized with further advancements in design. One port has been left open to adjust the setting according to the requirement. The conceptual design remains the same.

3.4   The Base Plate

This plate has a dimension of225 mm x 175 mm which is within the design constraint. It has a center hole for the shaft size of 25 mm.  This base plate has two things attached to it. One is the motion guide and the other one is the rotating part.

base plate

3.5    Pin

The pin has very small part to play but is kept away from the rotating part for two reasons .From manufacturing point of view as well as durability of the part, the separate inclusion of pin is important. This part attaches to the rotating shaft and forms a cam follower pair to guide the frame linearly.

pin

All the part models were made in Solidworks 2015 and were then assembled as per design. The part dimensions were all kept under constraints. The final assembly obtained was effective and has been discussed in the following topics.

4.     Final Model

The final assembled model has been shown below.

Assembled Model

The model was assembled with ease in the software. As it can be seen from above that there are two ports, each for inlet and outlet. These ports can be utilized in a dosing pump to periodically dose chemical. This assembly is a kinetic device which finally utilises a kinetic motion of yoke frame to feed the liquid at regular intervals but not continuous.

position when left port is open and the other is closed

In the above figure we can locate the position of the valve. Here the left port is open while the right one is closed thus allowing one type of fluid to flow and stopping the subsequent one. This can be termed as position 1 for the valve. The flow of another fluid has been stopped with the help of fram cum rod which doesnot allow the fluid to penetrate through.

left is closed and right is open

This is the second position of the device in which the second fluid is now allowed to flow through the port while the first one is stopped. The fluid in the first part gets a break with the help of the frame. Thus it can be seen how the fluid is controlled in both the ports and periodic dosing is done in both the cases.

5.     Application of the Kinematic device

The above made kinematics can find its application in many ways. As the project states it is dosing pump valve which allows two liquid to mix at regular intervals in equal quantity. (Anon., n.d.)

The main application of such device can be found in any Effluent treatment plant where two different effluent of acidic and basic nature are mixed to get the desired salt. In such plants this device can work amazingly. As seen in any Effluent treatment plant. Acidic fluid from one source is collected and basic fluid from other source is mixed in the tank such that their reaction produces a salt of pH 7. Here we have two inlet ports and their corresponding outlets. The flow of fluid is not constant but is constantly dosing from the outlet at regular intervals. Thus both the ports are being utilized simultaneously and the kinematic device has got its importance.

6.     Conclusion

The project gave a better understanding about the kinematic devices that can be used to obtain a suitable motion of the system . First of all the kinematics was discussed in detail to understand its application the design constraint and intent was important before proceeding for the design. A total of three cases were obtained and a case was selected for the design. Every part was made on Solid Works 2015 and was assembled there itself. The final product obtained was dosing pump valve which found its application in ETPs or Effluent Treatment Plant. The design could be modified for better uses. At last using reverse of Scotch Yoke mechanism a final product was obtained which can find its application in many dosing purpose.

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