New Chassis

[Catweasel]

I have just acquired what I believe to be an IP Punch chassis. As seems be a common problem is the white metal gearbox. Fortunately, it hasn't been glued so it was easy to see the problem. The spur gear is slipping on the axle. I know one guy drilled and pinned his, but I propose to strip the chassis down, clean the gear and axle with meths and rebuild using Loctite 603. As this will be a big beast, I'm going to put a 12 volt dry cell under the bonnet with radio control. Does this sound like a feasible plan?

[MDLR]

I use superglue to hold the pinion on the axle - had to do it on Esme this morning at Peterborough!

[Busted Bricks]

I have never had much luck with 603 on plastic gears. I usually bore them to be a very tight fit. CA glue works for a while but needs to be re-done from time to time in my experience.

[GTB]

Does this sound like a feasible plan?

The Loctite tech data sheet http://www.loctite.sg/sea/content_data/93772_603EN.pdf has a warning against using 603 for plastic gears. Loctite retaining compounds work by expanding in the bore as they set. Basically it forms a force fit in situ, but moulded acetal gears don't always take well to force fits and can split after a while.

Check your gear closely, it may be loose on the axle because it has split from when it was forced onto the axle originally. If so, Loctite, etc. won't work and the gear can't be repaired.

Superglue doesn't bond to acetal polymers and as Busted Bricks reported it will eventually let go under load. It sticks OK to the steel axle if it is clean, but the bond to the plastic gear is very weak.

Personally, I'd replace the plastic gears with metal ones if you want the model to perform heavy work.

Another point to consider is that the IP gearbox as supplied is only 16:1 reduction, which is a bit low for this sort of work. Especially if you put 12V into the nominally 3-6V IP motor (unless you are trying for the SM32 world speed record?).

For decent torque and scale running speeds in a diesel model I'd use a gear set with a minimum of 30 or 40:1 reduction. If you retain the IP motor, only use 6V as the power source.

Regards,
Graeme

[Catweasel]

Many thanks for your comments folks. Duly noted. At 16:1 the loco probably wouldn't frighten rice pudding skin so I'll see if I can source a gearbox.

[Busted Bricks]

I decided on 40:1 ratio in my kits as small DC motors don't have a whole lot of torque but like to rev. As for speed I'm happy running on 2 cells (3V) but on four cells (6V) it goes like a rat up a drainpipe.

Another reason why Loctite doesn't work well is that the gear would have been a push fit originally so fairly tight on the axle. Loctite needs a bit of a gap to properly seep in and harden. It bonds a lot better on parts that are a loose sliding fit.

As an emergency fix I have sometimes gotten away with squeezing the axle hard between pliers with serrated jaws. This creates something similar to a straight knurl and can help prevent slippage.