Need help with fixture design

[mgri]

Hi folks, I have a little design job that involves using a vertical reciprocating motion (i.e. a finger repeatedly pushing a button) to move a lead screw forwards incrementally. Once the carriage that is mounted on the lead screw is at it limit it needs to be returned. The picture at the link below describes my currect concept for achieving this motion. (Apologies, I had no success inserting this image).

As you can see, the vertical "actuation force" moves the rack down and drives the pinion gear clockwise, thus moving the lead screw forward. Upon releasing the downward force the spring returns the rack. The pinion is mounted on a one way bearing so when the rack moves back up the pinion freewheels and hence motion of the lead screw is the unwanted direction is avoided.

This issue I have centres around how to return the carriage that sits on the lead screw once it is al the way along the lead screw. You'll notice that in image the rack is not in contact with the pinion gear. This enables the lead screw to be rotated anticlockwise without the assembly jamming. Obviously it is undesirable to have the rack and pinion out of mating and may cause a clash of gears when engaging the rack and pinion.

Does anyone have an alternative way of allowing counter clockwise motion of the shaft to return the carriage without having the rack and pinion come out of contact to allow this?

I hope I've explained myself clearly, please ask questions if there's something I need to clarify.

Thanks very much all.

http://files.engineering.com/getfil...-3588-40ab-9b66-32f8dfa3d172&file=Concept.JPG

 

[kchan711]

There are various ways to actuate the turning of a screw. You could have a single direction spinning wheel that resets itself each time by spring. Or, better yet, have a contact gear that uses a small connecting rod to an actuator. In that case (similar to a crank shaft in a car), you achieve continuous drive on the screw. Like I said, ideas are plenty. This is probably something easier to sit down and chat about and discuss its application.

Regards,
Kyle

Kyle Chandler

http://www.chiefengineering.net

 

[mgri]

Thanks for your reply Kyle. I think what I need to focus on here is a mechanism to disengage the rack and pinion mechanism. I plan on looking at a system using dogs mounted on the shaft to do this. Any more advice would be great.

Thanks again.

 

[IceStationZebra]

You didn't mention the torque requirement, but you could use a magnetic or pneumatic clutch. Use a limit switch at the end of the travel to disengage the clutch and turn on a motor to "rewind" the lead screw. The problem with dogs is that they too can clash when re-engaging.

ISZ

 

[kchan711]

Yeah, that is a good question. What is the torque requirement?

Kyle Chandler

http://www.chiefengineering.net

 

[mgri]

The torque requirement would be minimal. This is a very low load application being driven by finger force in order to index an inspection fixture forward and back under a scope using a downward force only (lateral forces tend to knock the inspection fixture out of the field of view of the scope). I was thinking of using a dog clutch like the one pictured. The shape of this clutch minimises the chance of gear clashing. Problem is, I can't find anyone who make such a clutch.

Thanks for your thoughts guys.

 

[mgri]

Picture of desired dog clutch.

http://files.engineering.com/getfile.aspx?folder=e714d50b-8a4f-4afc-9248-b95092397276&file=Dog.JPG

 

[Bribyk]

I'd look at electric motors. Many have a ball bearing ratchet device so that they can't be run backwards (for pumps, etc..). Engine starters (air and electric) have a one-way clutch on the starter gear so that the starter doesn't run up to engine speeds once it starts. It's likely that a bearing company will have on-way clutches as well.

 

[patprimmer]

Some stud remover tools are sprag clutches. A very small one might do. Contact some automotive tool wholesalers or retailers.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[BrianPetersen]

Any particular reason not to use a servo motor controlled by a PLC?