Binding Gearbox

[garethashenden]

On a locomotive I'm building the gearbox is binding slightly and I'd like some suggestions on where to look to fix it. It is a High Level RoadRunner+ 54:1 with a Mashima 1220 motor. As the gearbox turns there is a noticeable tight spot. It's the same spot every time on the wheels. It seems like the sort of effect that is caused by a quartering problem, except it is still there with the rods off. I loosened the grub screw on the final gear and the binding is still there. It seems ok at higher speeds, but then I can see the motor shaft, the worm, and the flywheel all moving back and forth.
What sort of problem would cause this and where should I look? I've built High Level gearboxes before and they're been fine from the start, so I'm a little confused.
Here's what we're dealing with.

[Tim V]

It is a little late to be investigating this with a fully assembled chassis but hey ho.

First off, you say it binds when the grub screw is undone. OK , having done that take the motor off. Does it still bind? Have you looked very carefully using a high powered jewellers glass at the gear wheels for muck?

[Le Corbusier]

How is the worm gear fixed .... grub screw or lock tight?

[martin goodall]

I once had this problem with a Portescap-powered loco.

Having eliminated every other possible source of the problem, I found that there was still a cyclical variation in the speed and note of the motor. I naturally suspected that the grub screw on the final pinion was pushing the pinion off-centre. So I removed the grub screw altogether, allowing the pinion to turn freely on the driven axle. But the problem was still there!

So at last I had found the source - a final pinion which had been bored very slightly off-centre, with the result that it did not run absolutely true on the axle. The pinion was replaced with another one, and this finally resolved the problem.

Note that this was a Portescap gearbox, with a reputation for high quality. It just goes to show that even with the highest quality products, you can very occasionally get one with a minor manufacturing fault.

After that, I resolved to check every component of the running chassis as it was assembled, so as to avoid having to completely strip it down to find the source of the problem when testing the chassis after it has been fully assembled.

[garethashenden]

It is a little late to be investigating this with a fully assembled chassis but hey ho.

First off, you say it binds when the grub screw is undone. OK , having done that take the motor off. Does it still bind? Have you looked very carefully using a high powered jewellers glass at the gear wheels for muck?

I tested it before installation and it was good. However, I used a 9v battery so I didn't encounter the low speed problem. I'll try it with the motor off when I get home from work.

[garethashenden]

How is the worm gear fixed .... grub screw or lock tight?

The worm is held on by loctite.

[Le Corbusier]

How is the worm gear fixed .... grub screw or lock tight?

The worm is held on by loctite.

embarrassed by spelling again!!

[garethashenden]

Well I've taken a few things apart and put them back together, here's what I've found.
The motor is fine. It is smooth and consistent at all speeds.
The coupling rods/hornblocks/quartering is all good. With the motor removed and the grub screw loosened the chassis rolls quite well with the rods on. This is usually where I have trouble, so I'm glad it's not the problem.
If I hold the chassis in my hand and turn the flywheel I can't feel a tight spot. However, if the chassis is on the workbench and I run my finger over the flywheel (to turn it with minimum torque) there is a spot where it stops moving. This is where the grub screw lines up with the penultimate gear. It seems that the grub screw is making the gear eccentric enough that it binds with the gear next to it. This may not be the actual cause, but it is that sort of effect.
In the morning I will tear apart the axle and see if I can find anything definitive.

[Tim V]

Can you measure the axle diameter? Is it exactly 1/8", some axles I have are not that dimension.

[Andy W]

Try an extensive run-in before you strip it all back. Often an overly tightened screw can cause a problem, slacken all fastenings a tiny bit.

[Tim V]

I tested it before installation and it was good. However, I used a 9v battery so I didn't encounter the low speed problem. I'll try it with the motor off when I get home from work.

Use your usual controller to test the motor arrangement. I don't do this, using an old smooth voltage controller - no feedback control, so that problems are not masked by the controller. There is a variable controller using a 9v battery available - could be a good investment.

Did you test the complete gearbox - including with the grub screw up tight? Did you file a flat for the screw to press against?

I am minded of Chris Pendelenton's words in MRJ 28/29 - check at every stage, so you know when a problem occurs, it must be the last thing you did. All pre-assemblies must be checked at every stage.

[Philbax]

Before you take it all apart try:-
remove the grub screw from the final gear,
Carefully drill through the hole, without messing up the thread, into the axle and out the other side.
insert a piece of wire bent over at the ends to provide the drive connectin.

this will allow the gear to float on the axle and accommodate and out of centre on the gear.

good luck

Phil

[garethashenden]

Can you measure the axle diameter? Is it exactly 1/8", some axles I have are not that dimension.

Using cheap Chinese calipers the axle is 0.1255" all around. My employer would consider that wildly out of spec, but I think it's probably ok here. I didn't file a flat spot for the grub screw, should I do that in future?

[garethashenden]

I have made some progress with this. I took the axle out of the locomotive and removed the grub screw. I inspected everything as best I could and found nothing wrong. Everything seemed smooth. I put the grub screw back on but only snug rather than tight. I then ran the motor/gearbox/axle assembly outside of the chassis and noticed something odd. At one point in the rotation of the axle the whole axle would shift over suddenly. There are a number of fibre washers on the axle taking up most of the slack inside the gearbox but there was a little bit still there. I cut a gap in a new washer and stuck it in the space. Then I oiled everything and tried it again. I couldn't see any reason why the axle was moving over, but with the new washer it no longer did this. The gearbox was quite unhappy about this, almost stopping completely at slow speeds. It ran better at high speeds but with a slight bogging at that spot. I stuck it upside down on a block of blutack and left it for a few minutes, maybe 5. It was better after this but still very slightly sticky. However, in the other direction it was quite smooth. So I ran it that way for about 10 minutes. After that it was smooth in both directions. I'm going to keep it running separate from the chassis for the next few hours before I go to work. I'll change the speeds and directions a few times but I think it is on the way to being fixed. Thanks for you help.

[garethashenden]

One other thing. I've learned my lesson with regards to checking every step. I shall be more thorough in future.

[Tim V]

That "shifting" sounds like a problem with the gearwheel. The box should run smoothly, without having to "run in" for extended periods. Examine very closely the nylon gearwheels - may be some crud or incorrectly moulded teeth.

I had an engine that would run smoothly one way, but shuddered going the other - turned out the worm teeth were not moulded correctly. Change of worm sorted it out.

[garethashenden]

I agree that something isn't right. I can't see anything wrong with any of the gears though. It's rather confusing.

[Philip Hall]

Sometimes on a High Level 'box the gears run close to each other, brushing each other almost. I try to avoid this by arranging washers, very thin, to keep the edges of the teeth from contact. It may be that there is some roughness on the sides of the gears that touch once every revolution.

If the hesitation is at the same spot as the wheel revolves the problem is with either the wheel or the final drive gear, or (as above) a gear that it's in contact with. If it's more frequent then the problem is higher up the chain, from the worm downwards. As Tim says, High Level 'boxes should not need running in at all, and being mostly nylon gears running in would not wear anything off to improve matters.

I would also take a look through the 'window' by the side of the worm and revolve the motor very slowly by hand. Hold it up to the light and make sure that the daylight that you should see between the teeth is the same all the way round.

Philip

[Allan Goodwillie]

You are getting a lot of good advice here.

Could I mention something, if you should choose to get a new gear wheel from Chris at High Level that it would be a good idea to get a matching set as he has changed some of the parts in the gearboxes he is producing now. Any replacements for parts from older boxes people may have had for a while may require a complete set, rather than just a single part.

The West Lothian starters group we have going have been building locos for the first time and using the fourum as a place to discuss such things covers this very topic. https://www.scalefour.org/forum/viewtopic.php?f=39&t=4847&start=25 We came across this with two locomotive gearboxes where parts had been lost or gone missing and had to be replaced. There is also information on assembly and running in different makes of gearbox which may be of use to others following this thread.

Just because it is under the beginners section it should not be dismissed as I am trying to cover aspects of building locomotives in as comprehensive way as possible and getting the basics right. I always run in my gearbox / motor combination first before fitting it into any chassis. Checking everything as you go along is a very good mantra.

[Andy W]

"Checking everything as you go along is a very good mantra." Indeed it's vital, and not just at the chassis building stage. Running needs checking after adding the footplate; the splashers; the cab sides etc. I once built a loco where everything was fine until I bolted the entire, painted and lined body onto the chassis. Then it developed a tight spot. It was very odd. However, because of checking at each stage I knew it had to be the bolting causing the problem. I slacked off the bolt a quarter turn - and it ran like a dream. I must have imparted a slight twist on the chassis.

Had I not checked as I progressed I would probably have stripped the whole thing back - to no avail.

[Will L]

Sometimes on a High Level 'box the gears run close to each other, brushing each other almost. I try to avoid this by arranging washers, very thin, to keep the edges of the teeth from contact. It may be that there is some roughness on the sides of the gears that touch once every revolution.

I'm with Phil. Having read right through this I'd come to the conclusion that it may well be a rough spot on the side of the final gear catching against the gear next to it. I.e. it isn't a problem with the bits that you expect to work together all, only where the "smooth" side of two gears rub together. This fits with the push aside effect you noted. The fact that its once per rev of the final axle says it has to be the final gear. Phil's washer should do the trick, or take out the final gear and give that side a polish on fine wet and dry.

[Enigma]

I use a couple of methods of attaching the final gear on HL boxes. The first, for 'grub screw' versions, involves filing a flat on the axle, tightening the grub screw to make a centre mark, drilling and tapping 10BA (I think!), drilling out the threads in the gearwheel to 10BA clearance and then locating the wheel on the axle with a short length of 10BA studding. This alleviates the tendency of the grub screw to push the wheel off centre.

Where the gearwheel does not have a grub screw and to offer an alternative to Loctite fixing so that a disassembly of the box can be made later if required, I mark, drill and tap as above but then fit a 10BA cheesehead bolt with the head filed square into the tapped hole. A square
keyway is then filed into the axle hole of the gearwheel to fit the square bolt head and the gearwheel slid over the bolt to locate it. This works with the narrower HL gearboxes where the gears themselves and the box sides hold the gears in mesh.

Note that I do not claim any originality for the second method. The outline of it was described on RMWeb and I tried it, taking photos of progress as I went along.

In newer versions of some boxes where Loctite fixed gears were provided Chris now supplies a nylon gear with a fine brass tapped boss but I've not yet built one like this to see how they work in practice.

[Will L]

This is a picture of a RoadRunner+ and the arrow shows were I think the problem will be.

roadrunner.jpg (38.41 KiB) Viewed 4593 times

...Where the gearwheel does not have a grub screw and to offer an alternative to Loctite fixing so that a disassembly of the box can be made later if required, I mark, drill and tap as above but then fit a 10BA cheesehead bolt with the head filed square into the tapped hole. A square keyway is then filed into the axle hole of the gearwheel to fit the square bolt head and the gearwheel slid over the bolt to locate it. This works with the narrower HL gearboxes where the gears themselves and the box sides hold the gears in mesh...

Interesting idea, just ensure that where the final gear runs against the side of the previous gear as in the RoadRunner+ that the bolt head can't catch the teeth of the adjacent gear giving you a rather worse varsity of the problem he already has.

[Andy W]

Very neat enigma. Will's right about making sure there's no fouling from the gear's side. It's a good idea to place all gears and washers side down on some very fine wet and dry and rub them around with your finger to get them as smooth as possible before using.

[Enigma]

This is a picture of a RoadRunner+ and the arrow shows were I think the problem will be.
roadrunner.jpg

...Where the gearwheel does not have a grub screw and to offer an alternative to Loctite fixing so that a disassembly of the box can be made later if required, I mark, drill and tap as above but then fit a 10BA cheesehead bolt with the head filed square into the tapped hole. A square keyway is then filed into the axle hole of the gearwheel to fit the square bolt head and the gearwheel slid over the bolt to locate it. This works with the narrower HL gearboxes where the gears themselves and the box sides hold the gears in mesh...

Interesting idea, just ensure that where the final gear runs against the side of the previous gear as in the RoadRunner+ that the bolt head can't catch the teeth of the adjacent gear giving you a rather worse varsity of the problem he already has.

I think I got away with it OK on this little beastie (!! - read the thread to see my labours - http://www.rmweb.co.uk/community/index. ... arclay-16/ -) It runs very nicely with no trace of binding.