How to ensure smooth engagement of moving pinion with fixed rack? 3

[marvincooper]

Hi

I have a pinion which is being carried towards a rack. The pinion is turning. I want the pinion to engage smoothly with the rack without stoping the pinion.

Have thought about mounting the pinion on a shaft split with some sort of flexible coupling.

Also what if I tapered off the ends of the rack?

Any other ideas please, anyone done this before?

Thanks
marv

 

[Tmoose]

http://www.supersplit.com/principle.htm

 

[ExRanger]

Crazy idea - what if the rack itself had some play in its travel before it hit a hard stop. In other words, if the whole length of rack could translate +/- one pitch length freely (or more likely with some small resistance) before it hit a stop. Then if a pinion tooth came crashing against a rack tooth it could move over to allow proper engagement at which point the pinion would drive the rack hard over to its stop for normal operation.

 

[marvincooper]

MikeHalloran:
I can't extend the fixed rack - that's why I have this problem. Thanks anyway.

SincoTC:
Thanks again. It does sound like a similar problem though I think I have much higher forces to deal with. I don't really have room (I don't think) for a pinch roller or a large drive wheel, but will have to explore this and see what the implications are on the rest of the machine.

ExRanger:
Yes, that's an idea I had too but there is still the problem of the pinion teeth whacking into the rack. Plus I don't really have space to add more bearings. But it may be worth considering so I will have another think on this. Thanks.


marv

 

[hydtools]

Pivot the far end of the rack. Put a spring under the engagement end of the rack. When the pinion approaches the rack, it will deflect away letting the pinion find an engagement.
Or reduce the length of support of the rack and take advantage of the flexing of the rack instead of installing a spring.
No idea of the dimensions of your rack as to which idea would be better or no good at all.

Ted

 

[FeX32]

I would go with Mike Halloran's idea.


Fe

 

[gearcutter]

I hope this makes sense.
Split the rack so that you have a pre determined "lead in" or approach length.
Allow this length of rack to pivot off the main rack length in a way so as to maintain the pitch without too large a variation. The longer the “lead in” rack length is the less change in pitch as the length pivots.
Mount a spring on the opposite end of the pivot point or right at the start of the rack assembly. The spring is placed under the rack and should "give" if teeth tips come into contact but provide enough force to resist the radial separation forces of the mesh. You could maybe even use a hydraulic system to replace the spring. Either way, design the system so it is adjustable.
It seems to make sense to use a high pressure angle in this case but I think that a lower pressure angle might be best as this would reduce the separation forces and actually help draw the pinion into mesh. A lower pressure angle would also allow for wider top lands of the teeth making them stronger and less prone to chipping etc.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[marvincooper]

I would love to go with Mike Halloran's idea, but I can't extend the rack and the torque will vary a lot depending on different installations, so a clutch won't work (I don't think).

gearcutter:
I'm liking the sprung pivoting rack idea, will look into this more. I guess there may still be a nasty "clunk" if the pinion hits the rack just wrong but it may be worth a try. Thanks.

 

[dvd]

M. Cooper

It may be time to consider other concepts besides the gear rack. Why don't you post a decent sketch, so that others can see what you're up against?

 

[ExRanger]

Marvincooper:

In conjunction with an idea like the spring-loaded pivoting rack or sliding rack, could you make the first few rack teeth out of hard plastic or rubber? Maybe something like PEEK. I don't know how much torque you will have, but if its low enough that the plastic teeth can survive and won't deflect excessively, then it would provide a smoother engagement. I would guess that when the carriage first engages the rack, the carriage wheels will still provide some of the drive momentarily, plus you have some forward momentum of the carriage. Maybe the first 2 or 3 rack teeth will never see the full torque anyway.

 

[MikeHalloran]

Marvin, you've got everyone shooting in the dark.

So far you haven't revealed squat about how big this drive has to be, or what forces or torques or masses are involved, or what level of precision is necessary.

You could meet us halfway.


Speaking of halfway, if the pinion is geared to spin at exactly "rack speed", then it will stay almost in sync with the rack over a fair distance of travel away from the rack, and re-engage without much fanfare, provided that the carriage wheels don't slip much.

If on the other hand, the wheels do slip, or the carriage is otherwise started toward the rack while randomly out of phase with it, then the chance of a clunk- free engagement is roughly proportional to the ratio of pinion lash to tooth thickness. I.e., mostly clunky.


So I was thinking of rack teeth that are flexible, not up and down/ in and out of mesh, but in a pitch-wise direction, e.g. formed at the tips of a comb-like structure, perhaps with deeper saw cuts between the teeth at the free end, etc.

Or, you could use a pinion that doesn't really much care about rack phase, e.g. a wire brush.






Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

Kinda sorta like Mikehollran's suggestion about extending the rack. Have a rack on the part that is moving the pinion in and out. Have it where the moving rack will make up to or match the stationary rack where the pinion will keep moving and be in-sink with the stationary rack.

 

[hydtools]

Side shift the pinion into the rack. Chamfer the tooth edges of the pinion to ease the pinion lead-in to the rack. The pinion should be in sync within one rev as it goes by the rack and will engage the rack. Could be a cam operated shifting fork that pushes the pinion sideways into the rack. The pinion would need a sliding fit on its driving shaft.

Ted

 

[plasgears]

Exranger, with his idea of limited linear movement of the rack, reminds me of lawn tractor starter motor engagement. We wrestled with lack of good starter engagement and hit upon two fixes: tooth entry relief and wobble clearance in the motor pinion. This fixed the problem. Hope this helps.