Gearbox restraining

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petermeyer
Posts: 310
Joined: Tue Oct 01, 2013 10:06 am

Gearbox restraining

Postby petermeyer » Tue Jan 18, 2022 1:22 pm

Fri Jan 14, 2022 12:24 am

petermeyer wrote:
...I've missed any discussions on gearbox restraint recently but was under the impression that it was not an issue with a fixed axle...

Will L wrote:
That's right,its only an issue when the driven axle is free to move up and down on some form of suspension.


I’m building a couple of Finney 4-4-0’s at the moment with twin beam compensation so no fixed axles. These were designed around Portescaps that are now defunct but appear to have restraints built in. Will I need to devise a restraint for the replacement High Level gearboxes?

davebradwell
Posts: 1174
Joined: Fri Jan 18, 2019 3:48 pm

Re: Gearbox restraining

Postby davebradwell » Tue Jan 18, 2022 2:02 pm

That depends on how sound the original restraints were!

DaveB

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Jol Wilkinson
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Joined: Mon Jul 21, 2008 7:39 pm

Re: Gearbox restraining

Postby Jol Wilkinson » Tue Jan 18, 2022 2:28 pm

Peter,

the "drivetrain" on an axle mounted drive system needs some sort of restraint to prevent the whole lot rotating around the axle. Because the axle will also be able to "rock" slightly from side to side in a twin beam system the restraint must have an amount of lateral movement. A piece of 1.0mm wire loosely attached to the chassis and gearbox longitudinally (i.e. through a hole in a chassis spacer and one on the gearbox) should achieve that. Alternatively a strap around the motor, loosely bolted to a crossmember.

With a fixed axle drive I use servo tape - very sticky but slightly spongy - to do the same thing as it also provides a bit of noise insulation. A small plate with a fold up tab, drilled to take the retaining wire/strap and stuck to a crossmember with servo tape would be one way to do it.

petermeyer
Posts: 310
Joined: Tue Oct 01, 2013 10:06 am

Re: Gearbox restraining

Postby petermeyer » Tue Jan 18, 2022 6:19 pm

Thanks Jol,

There is a hole on the bottom of the spacer nearest the cavity for the motor so I guess I can use that with a suitable piece of wire attached. I doubt that I can use the parts designed for a Portescap to harness it as they are quite different to a High Level gearbox. My High Level Dean Goods chassis has such an arrangement so I'll mimic that.

Peter

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Will L
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Joined: Sun Jul 20, 2008 3:54 pm

Re: Gearbox restraining

Postby Will L » Tue Jan 18, 2022 6:23 pm

petermeyer wrote:I’m building a couple of Finney 4-4-0’s at the moment with twin beam compensation so no fixed axles. These were designed around Portescaps that are now defunct but appear to have restraints built in. Will I need to devise a restraint for the replacement High Level gearboxes?

We are talking Torque Reaction links here. Search the forum for that you will find it has been covered on her quite often.

There is nothing special about a Portescaps and they can need a torque reaction link link just like everybody else.

On a fixed axle you may want to stop the motor/gearbox revolving around the axle. If you don't it will hit in inside of the body somewhere but the audible bonk you get before the loco moves will be the only effect.

You should do something for any chassis on which the driven axle is free to move up and down. Theory wonks like me worry about this sort of thing but many people don't and often they get away with it, although strange things they can't account for may happen. But these sort of things don't get talked about in polite society and locos that are given to lifting a leg for reasons they can't explain tend to end up on shelves looking nice.

The theory says that the motor/gearbox on a axle which isn't fixed rigidly in the chassis should be restrained so it can rise and fall vertically but not rotate round the axle. This restraint should act horizontally along the chassis, or it can adversely affect the driven axle. A wire link from a point on the gearbox directly above the axle to a fixed point on the chassis at exactly the same high as the point on the gearbox and pivoted at both end is one good solution. A vertical tab on top of the gearbox, again directly over the axle, working in a fixed slot in the bodywork is also used, (although Ted worries about friction between the tab and the slot, a which is a logical possibility)

All axle compensated chassis are less sensitive to this sort of problem than a sprung chassis but it is possible to get odd things happening if you don't get the torque reaction" link right and the chassis is a bit stiff.

You can probably deviate from an ideal solutions on an all axle compensated chassis without issues so long as you have a nice free running chassis. It will affect the amount of traction on the driven axle but unless something really nasty is going on, or your asking for every last gram of pulling power, you'll probably never know the difference.

A sprung chassis is a bit more sensitive and strange things can start happening if you get it wrong.

petermeyer
Posts: 310
Joined: Tue Oct 01, 2013 10:06 am

Re: Gearbox restraining

Postby petermeyer » Tue Jan 18, 2022 7:13 pm

Thanks Will

I’ve been wondering, with my wasp-waisted pre-grouping locos, as there is little room betwixt motor/gearbox and inner body whether that’s enough of a restraint. And you’ve answered why I seem to have got away with it thus far.

When these gearboxes arrive this time I will give it a go.

davebradwell
Posts: 1174
Joined: Fri Jan 18, 2019 3:48 pm

Re: Gearbox restraining

Postby davebradwell » Wed Jan 19, 2022 9:30 am

This is a far more important topic than many realise and deserves careful consideration. Some of my early chassis were transformed after the penny dropped. Ideally (which should mean every time) for starters you are trying to avoid the torque reaction lifting/lowering the axle on a loco with suspension rather than this job being done solely by the track. so vertical tabs and horizontal torque links This bit is relatively easy to understand.

The next consideration is where it goes wrong for many, that is avoiding the restraint restricting any other movement. It's basic engineering that you don't over-constrain things - 3 bearings on a shaft is going to cause trouble and it's easy to make a restraint that does too much. A torque link must have very loose pivots to permit all other movement. The tab must fit in a slot with plenty of clearance at the ends. Even with a so called rigid axle the rules are exactly the same, the axle must be allowed to float to its own position in its bearings without external influence. Even a rigid axle moves about, especially after it's worn a bit and will try to take up different positions when running forwards or reverse. This is why just letting the motor flop about inside the body works quite well - apart from the clonk it's a good compromise despite the vertical component. Was it Ted S who recently posted a warning about it, though?

Whatever device you use should be on the loco centreline to avoid a tendency to twist the gearbox when it's doing its reacting.

Finally in our quest for reliable slow running, the electronics folk give us pulsed controllers of assorted quality which give everything a good shaking so it's necessary to consider how this will affect those pivots that need to be loose. Perhaps fit rubber bushes as auto engineers do.


Just to add a further issue - do you know what is taking the end-thrust of the worm in each direction? Another area with a potential for disaster.

Cheers,

DaveB


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