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

 

[eromlignod]

Can you link it with another pinion that is already on the rack (like with a timing belt) and synchronize it?

Don
Kansas City

 

[marvincooper]

Unfortuately not. The pinion cannot be linked to the rack once it has disengaged; it needs to be disengaged completely. Thanks for the try!

 

[Timelord]

Perhaps a cam that aligns the pinion to the rack as it draws close. Do you care if the pinions motion is controlled somewhat prior to engagement?

Timelord

 

[marvincooper]

No, don't think that would work. I'd better explain the prolem in more detail.

The pinion is on a carriage. The carriage drives itself along with a motor which is linked to our pinion. So when the carriage moves, the pinion is turning.

the carriage drives itself up to the rack. The turning pinion then has to engage with the rack, as smoothly as possible, with no stop/start to the carriage motor.

There is no way (that I can see) to maintain the pitch of the pinion in sync. with the rack as it's movement once it is off the rack is independant.

Hope this makes some sense, and many thanks for the ideas so far.

 

[SincoTC]

Could you have the driven pinion on a swing-arm, spring biased towards an engaged position stop, so if it encounters a jamming situation it can ride up, the continuous turning force would tend to pull the rack in until a meshing condition occurs, and then it would come back to the bottom stop. It may require a damper to avoid slamming down and possibly a latch to keep it in mesh if the torque is high.

 

[hydtools]

Mount the pinion to a slip clutch so the pinion can slow/stop as it engages the rack. As the rack moves and increases speed the gear will be brought up to the speed of the drive shaft.

Ted

 

[gearcutter]

Will the pinion be under load once it has engaged with the rack?

As the pinion is connected to the motor, can I assume that once it engages with the rack it then becomes the driver?

I think you’ve already covered this but I’ll double check; after having meshed with the rack and then disengaging will the pinion maintain the same angular position relative to the rack?


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[ExRanger]

You could have a kind of ramp that the carriage could drive along. The ramp would be parallel to the rack. As the carriage drove along it would start on the decline of the ramp with the pinion positioned above the rack. As it went down the ramp the pinion would be brought down radially into mesh. If the rack is moving, you'd have to synch the speed of the carriage to the rack so that the pinion was rotating at the correct speed to mesh. If the rack is stationary, just bring the carriage in slowly. Then you could make the pinion shaft have some rotational compliance to it (like a quill shaft) so that it can take up the phase difference in the meshing teeth.

You could make the teeth thin with a lot of backlash or have a high pressure angle (say 25 degrees) with thin toplands.

 

[marvincooper]

Firstly, many many thanks for these great ideas. This forum has been much more useful than I had thought it would be!

So to reply to you all:

To SincoTC:
For my application, I think the swing-arm mounted pinion may have some mileage but I am worried that if the pinion hits the rack at just the wrong point then it could end in broken teeth or a stalled motor. If you have any real-world experience of making this kind of thing work I would love to know more.

To hydtools
I had considered using a clutch on the pinion shaft, however there is a large variation in torque that the pinion will see so I doubt it would work in all load scenarios.

To gearcutter:
Yes the pinion is under load and is the driver when engaged with the rack. I can’t maintain it’s mesh with the rack once it has moved away though.

To ExRanger:
A restriction on the design is that the carriage can’t be raised or lowered relative to the rack. Thanks for the suggestion of a quill shaft, I have never seen one of those.


Thanks all. I’m thinking I may use a drive tyre instead of a rack and pinion……

 

[hydtools]

marvincooper, the drive tire would behave like a slip clutch. Your best choice may be a friction drive rather than a toothed gear drive.

Ted

 

[marvincooper]

hydtools, what do you mean by a friction drive? A drive tyre would be just that wouldn't it? Or were you meaning something else?
ta
marv

 

[SincoTC]

Sorry; plenty of experience with fiddly/complicated friction and gear driven mechanisms, but nothing directly analogous to this!

I would have thought that providing the pinion is always being driven in the same driving direction as the carriage and it is not permitted to drop below the full engaged depth relative to the rack, then it would always be able to "climb out" of a jam/stall situation. I would be inclined to truncate the first rack tooth to about half height and chamfer the inner flank back like a buttress thread to aid engagement and strengthen the tooth.

My biggest concern (afterthought), was that if the carriage was being driven at nominally the gear-driven rate, there would be no speed differential to synchronise the meshing, so if it climbed up tooth top to top, it would just roll all the way along the tops like this, not a lot of use! However, if the carriage drive was slower and the pinion could PULL the carriage at its slightly faster speed, then it would all work OK, however, as you said that the drive was shared, this would mean the wheels would have to skid when the gear was driving (unless they were driven by over-run sprag clutches or similar).

If you are considering a drive tyre, I would recommend a vertical blade in place of the rack and have a vertical spindled drive wheel opposed by a pinch wheel so that your driving force is independant of actual weight on wheels, which, with a vertical wheel would be in conflict with the other carriage wheels, plus it will give you directional control.

 

[marvincooper]

SincoTC:
There is an intermediate gear between the carriage drive and the pinion, so the pinion moves faster than the carriage. So the pinion will indeed pull the carriage - this is the intention.

So, you reckon that if I chamfer the rack and drop the first tooth height, it would mesh OK? This was how I planned to overcome the meshing problem but I was not confident it would work, and if it did then I worried there would be a noticable "clunk". What do you think?

Another idea I had was to use a static flat chain instead of the rack and to use a sprocket instead of the pinion. I thought the sprocket might ride up / engage in a more forgiving way. Any thoughts on this?

 

[SincoTC]

Yes, I feel that the with the first tooth modified like this, it moves the "worst case" first contact nearer horizontaly to the pinion's centreline, bringing line from this point to the pinion axis nearer to the vertical so that it lifts the swing-arm and reduces the bending moment on the rack tooth as below:

http://files.engineering.com/getfil...0fa-058af1ebe7e4&file=rack-pinion_example.jpg

There isn't much grip between pinion and rack in this situation, but with the pinion rotating and the carriage driven at a slower rate it should come to a point where it will haul itself onto the rack. How well and quietly it achieves this is difficult to judge. A lot depends on the mass and size of the parts involved, how fast the pinion is turning and the strength of the swing-arm spring, which is a balance between strong enough to maintain mesh when driving, but easy enough to lift up out of trouble on initial meshing. I would fit an hydraulic shock absorber/damper to prevent it slamming down. My gut feeling is that if the speeds are relatively slow, there would be more chance of success.

However, I would definately make some kind of mock-up or scale model to experiment with first, just to make sure it works OK and there are no show-stoppers! Sorry, I've not had any experience with chains, other than that sprockets are usually narrow pointy things that get burred and damaged relatively easily.

Good luck!

 

[djm883]

Don't know if this is possible, but if you were concerned with improper tooth contact causing the pinion to jam with the rack, could you position a prox. switch to signal where the pinion should stop?

You should know the speed and the amount of time/distance until the pinion engages the rack. If you know what position of the pinion tooth you want to engage the rack, determine the position the pinion needs to be stopped at (or where it starts from). Place a prox. switch to make sure the pinion is oriented in that manner and allow the prox. switch to determine when to stop the motor when coming to a rest. Since you know the speed and distance to the rack, you could position the pinion so it engages at a certain point on the rack teeth.


Just a thought.

Don

 

[hydtools]

marvincooper, I was saying that your tyre drive would be a friction drive which may be more appropriate than the gear drive. The screech and squeal may be distracting the first few times the tyre contacts the drive surface, depending on the tyre speed at time of contact and relative carriage speed.

Ted

 

[marvincooper]

SincoTC:
Thanks again, sounds like a good plan. I'll give a prototype a go(if I get the chance, depends on client reaction) and see how it goes - will post results here. Might be a few weeks though.

djm883:
Sorry, won't work as I can't stop the pinion rotating.

hydtools:
I have the same thoughts and would like to give this a try. If I get the chance, will post results of prototype here.

Thanks all!

 

[MikeHalloran]

Extend the rack over the entire travel of the carriage.
Leave the pinion in contact with it all the time.
Use a clutch on the pinion shaft.
Make it a dog clutch with balk rings, just like in a synchromesh transmission.

... Which is something you should study. All this stuff with chamfers and weird teeth was studied, tried in production, and found wanting in the last century.

The other choice is some fairly fancy electronic controls.



Mike Halloran
Pembroke Pines, FL, USA

 

[SincoTC]

MikeHalloran:

I agree that on their own, chamfers and weird teeth have proven to be of little or no value, but in combination with a mechanism that allows the pinion to move away from the correct pitch distance, we found that they will substantially reduce the load on the rack's leading and second tooth.

marvincooper:

I was thinking overnight about the relevance of the device I worked on had to your problem, so I will explain what it was and you can form your own opinion.

Quite a few years ago, we were approached to help sort out a piece of Lab equipment that had become unusable and the originsal manufacturer had eventually refused to help. It consisted of several small interchangeable steel blocks that slid in dovetails on a microscope stage. A short length of rack was fixed to the stage between the dovetails and was engaged by a thumbscrew driven pinion in each block, this enabled the blocks to be precisely positioned under the microscope. The parts were quite well made, but the gears were small at 96DP; with the pinions mounted rigidly and on the tight side of the correct centre distance, they needed care and a little "engineers feel" to engage smoothly. This all went OK until an eminent Professor took over the lab and wanted to do some experiments himself. He was a brilliant man, but totally lacked the aforementioned "engineers feel", in fact, his lab technician said that his Prof would break a hammer if he was daft enough to give him one! In under a month, all the gears were in a terrible state, with bent rack teeth and broken pinnions, the manufacturer replaced the gears under warranty and returned them. However, they still jammed, so the lab tech chamfered the rack ends. I actually think that this didn't help the engagement and more likely made it worse! Within a week, all the gears were mashed again and this time the manufacturer told the lab to go away (because they had been tampered with) and that's where we came into the picture.

Unfortunately, other reasons prevented a full length rack, so we were stuck with the short rack drive because a friction drive that would fit in, couldn't cope with the static friction of the dovetails. Therefore, we renewed the gears and mounted the pinion bearings on flexure strips, these permitted enough movement to allow the pinion to climb out of a jam situation and their compliance allowed a tight but smooth mesh. We made a dummy block out of perspex so that we could watch the engagement action with another microscope and came to the conclusion that although it seemed to work well enough with plain ends to the rack, the stubby shape (shown in my last post) protected the first normal tooth from the worst case jam and gave a better lifting action to the pinion. They've been back in service now for over ten years and I hear that they still work very smoothly and that anyone can use them without difficulty.

So, now you can see that although the rack drive part is similar, I expect that there is a vast difference in scale and other details, which when added to the mechanical drive leads to my element of caution.

I do have to ask the question, why do you need the rack drive? We were stuck with it by the required precision, limited traction and lack of space for a pinch wheel. The method we used may well work in your case, but unless the rack is essential or can't be extended to full travel for some reason, then I would think that a friction drive probably has a more predictable outcome and easier development.

Good luck again