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cenderos
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Rack and Pinion

05/13/2011 5:03 PM

i m going to use rack and pinion for power transmission in tensile testing system.I have a stationary rack. So, the linear motion should of the mechanical grip which holds the specimen should be obtained from the rotary motion of pinion. Pinion should be connected to grip and motor in some way. Our linear speed should be 3 mm / min, and the required force for the failure of specimen is 4000 N. How can i do engineering calculations? what should be used for connection of the grip and pinion?
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RVZ717
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#1

Re: please help me:) Rack and pinion

05/13/2011 5:13 PM

I would use hydraulics... but that's just me.

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34point5
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#4
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Re: please help me:) Rack and pinion

05/13/2011 7:09 PM

Yeah, but that's because you want an accurate and repeatable result, rather that one subject to the radial migration of the tooth form contact and friction variables of 'stray vectors'

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#2

Re: please help me:) Rack and pinion

05/13/2011 5:17 PM

You have a mechanical rack and pinion gear assembly where one turn of the pinion gear should move the rack a known distance. Now you say that you do not know at all how to calculate any of the forces involved. Well a Google search of Sir Isaac Newton might help you. I also recommend that you review the topics of static and dynamic friction.

However if these terms and ideas are new to you, you should put the pencil down and take some basic classes at your local community college.

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#3

Re: please help me:) Rack and pinion

05/13/2011 7:07 PM

Don't know how critical your test results need to be. But I'm with RVZ on using hydraulics to provide the force. The binding in the gears of a rack and pinion will have to be figured in. To many unknowns there.

Hydraulic cylinder to provide the linear force can be slowly regulated with a valve. A load cell on the stationary side gripper would give a very accurate reading. Even with out the load cell a gauge on the fluid pressure maybe accurate enough depends on your needs of test accuracy.

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#5

Re: Rack and Pinion

05/14/2011 2:20 AM

I've never seen a tensile testing system using rack and pinion, they are normally hydraulic or a screw thread. As others have said the engagement of the teeth cause to many variables to give a reliable output.

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34point5
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#7
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Re: Rack and Pinion

05/15/2011 12:53 AM

It occurs to me 4000 N is about 400 kg, or the sort of force a lecturer may deem 'safe'.

Also no mention of plotting 'elongation' is made.

It could be 'hydraulic' and/or 'screw' is also deemed 'prohibited', so they have to do the 'rationale' as to "why it is so".

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rcapper
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#6

Re: Rack and Pinion

05/14/2011 11:24 PM

Sounds like an exceptionally bad idea. As noted, hydraulics or a screw would greatly reduce the strain on the machine and lower the torque requirement of the motor. Even something with a lever maybe and a cable to a take up reel. Go back to the drawing board and think this out several ways before you get too invested in one way.

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cenderos
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#8

Re: Rack and Pinion

05/15/2011 2:33 AM

Please do not suggest any other power transmission type. This is a project in the school. Rack and pinion must be used. I do not know how i select pitches of gear and rack. Also i dnt know how i provide connection for gear and grip to test the specimen
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34point5
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#9
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Re: Rack and Pinion

05/15/2011 2:50 AM

Ok - now you have 'illuminated': think of having the rack moving and the pinion fixed.

This is much more compatible with a 'gripper solution'.

The rack will move 1 tooth per pinion tooth. But if you go below 9 teeth on the pinon you get distortion in the tooth form and a weaker root strength.

Though a straight tooth is stronger, a helical form will be 'smoother', so more consistent in resultant lineal velocity.

The above gives you your basic 'start point' to exploring the mechanics.

More than that is an unfair advantage and 'gifted answers' do not 'teach' what you are supposed to learn by doing this exercise.

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#10
In reply to #9

Re: Rack and Pinion

05/15/2011 11:21 AM

GA & a bravo. Exactly the type of help we should be giving a student. A warning of a subtle complication combined with a good staring point for them to do their analysis.

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34point5
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#11
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Re: Rack and Pinion

05/15/2011 2:00 PM
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RVZ717
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#12
In reply to #9

Re: Rack and Pinion

05/18/2011 11:15 AM

Agreed, GA.

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34point5 (4); cenderos (1); ozzb (1); rcapper (1); redfred (2); RVZ717 (2); TonyS (1)

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