Rack and pinion efficiency

[Martin DowingParticipant @martindowing58466]

What level of force can be produced by turning a carriage driving handwheel in pinion -rack system of ML7 assuming that an effort is kept within reason, not an attempt to break something?

Alternative question: what sort of torque (in N*m) one can deliver by hand by twisting action like turning a handwheel of lathe if effort is kept within reason? Answer should help to calculate answer to first question.

Martin

[Martin DowingParticipant @martindowing58466]

[Neil WyattModerator @neilwyatt]

Well you can probably apply 100 newtons (22 pound force) without diffculty and keeping yourself stable.

The force at the tool will depend on the intermediate gear ratio times the ratio of handle diameter to the diameter of the final pinion (as racks don't have a gear ratio as such).

Neil

[SillyOldDufferModerator @sillyoldduffer]

The question's too open-ended for accuracy but experimenting with a set of bathroom scales I find I can press down 20kg without exerting myself unduly. That's 200 Newtons rather than Neil's 100N but we're very much in the same ball-park.

The diameter of the hand-wheel gives a mechanical advantage, say 4:1. Neil's 100N becomes 400 and my 200N becomes 800.

The gearing between hand-wheel and rack provides a further mechanical advantage, on my machine 10:1. That puts us in the range 1000N to 8000N

As I weigh 90kg (oh dear), that gives the maximum force I could apply to the handle by using it to lift myself off the ground, say 900N x 4 x 10 = 36000N

A lathe fitted with power-feeds (big motor with much gearing down) would apply much larger forces – enough to break the machine. That's why they're fitted with shear pins.

Quite an interesting question to ask of a lathe is: 'what will break first if I overload it?' Ideally it will be something quick and cheap to replace. Mini-lathe owners sometimes replace the plastic gears with metal. Not convinced that's a good idea: apart from the increased mechanical shock a stalled motor might fry the electronics.

On my Chinese lathe, the first thing to break is the shear pins. (Don't ask how I know.)

What breaks first on your lathe when it's abused by an angry gorilla?

Apologies in advance if my maths is wrong. I offer the answer partly in expectation that some kind person will correct me! Again…

Dave

 

Edited By SillyOldDuffer on 05/03/2018 11:50:29

[larry PhelanParticipant @larryphelan54019]

Dave,did you break your shear pin too? Welcome to the club !!

[not done it yetParticipant @notdoneityet]

Ratio of effective diameters will provide the mechanical gain. It will not be more than that.

[Martin DowingParticipant @martindowing58466]

Hi,

Thanks for comments,

Handwheel in ML7 has effective diameter of ~75mm what gives circumference 235.5mm, say 250mm. "Effective" means diameter circled by handling knob.

One turn of wheel will give you 20mm of carriage travel.

This implies overall gearing ratio 12.5:1

So one should get 1250N (~125kG) force according to Neil or 2500N (~250kG) according to Silly Old Duffer.

We should take off 20% to address resistance on friction of slide and gears, so we are left with 1000-2000N (~100-200kG).

Probably 100kG because it is much easier to push against balance than turn a wheel.

I am designing bed stop with a relay system to turn off motor upon hitting it. Business end must have a spring loaded bumper to provide stopping distance to address pulleys and chuck inertia, however resistance should be sufficient so the bumper doesn't retrive during normal hand operation and stop is felt as such.

So I am calculating how much preload need to be applied to bumper springs. Stack of disc springs will be used.

Anyway, thanks for ideas regarding reasonable torque of hand.

Martin

[Russell EberhardtParticipant @russelleberhardt48058]

I'm not sure that the force that can be applied by pushing or pulling is relevant. Rotating a handwheel uses a different combination of muscles.

I don't have a Myford but when I put a spring balance between the tailstock and the toolpost on my Atlas (similar design to Myford but a bit bigger) I could get up to 40 kgf. I am only 56 kg and in my 70s so you can probably double that for a bigger, fit young man.

Russell.

[norman valentineParticipant @normanvalentine78682]

My Loughborough lathe has rack and pinion drive for the saddle but I found, when cutting 3mm wide keyways that the force required was enormous and I quickly gave up and made a slotting attachment that does the job without any strain on the lathe mechanism.

The point of my reply is that I think that it is possible to put so much force into it that something is sure to break, so avoid it!

[Martin DowingParticipant @martindowing58466]

Many thanks for comments.

It seems that on a small lathe like Myford we are talking about 100kG or 1000N of force while working reasonably.

So now I can buy these disc springs for stacking up "bumper spring". They are not very cheap so it is best to buy right ones at first go.

@Valentine,

Lever or hydraulic press and a broaching set is a right way to go with that. Or save some old good shaper from a scrap if you only have a space to keep it.

Martin

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