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.