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Chassis Problem. Help needed.


MerseyMan

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Hi Guys. I just completed soldering up my chassis for a Ivatt class 2 and although it sits completely level on a plate of glass, there still seems to be a tight spot once the coupling rods are fitted. The wheels run smoothly with no tight spots at all on there own and even when the front set of coupling rods are attached. The problem seems to be once I fit the rear set of rods. Could anyone offer the best method to find and to rectify this problem.

 

Many thanks

 

MM.

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MM

 

Using a Broach open up the holes for the rods on both sides so that they are a sloppy fit on the pins and then try it, you don't need engineering tolerances on the rods when you are making a model you will find that it should run a lot better.

 

Pete

 

 

I agree with Pete. 1)Check that your crank pins are square. 2)Broach out all the coupling rod holes to give 0.5mm of slop. That ought to do it unless your bearing relationships are way,way out.

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MM

 

Using a Broach open up the holes for the rods on both sides so that they are a sloppy fit on the pins and then try it, you don't need engineering tolerances on the rods when you are making a model you will find that it should run a lot better.

 

Pete

Be very wary of doing this. Too much and you will cause more problems.

Only do this if the rods are actually binding on the pins.

 

Is the tightness on one side or both sides?

Try the rear set on their own, do you still have the same tightness?

 

A common cause can be:

1. the pairs of rods are not identical,

2. the axles are not perfectly parallel in the frames,

3. the crank pin(s) are bent.

 

Poor quartering is fairly difficult if the above are perfect and would show up as a lolloping motion (easier on one stroke than the other) rather than actual binding.

 

If it is only very slight, tightness that is easily overcome with the motor+gearbox in place, the best solution may be simply to add toothpast or T-cut and to run the mechanism in.

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Hi Guys. Thanks for all the replies. I’ve had another look at it tonight and tried it with just the rear set of rods on. Strangely enough it runs nice and smooth with no tight spots at all (just like when I run it with just the front set on). I’ve also noticed when everything is loose (Nuts are not tight on the crankpins) it runs a lot better. However once everything is tightened, it really starts to jam up. Ive also noticed the crankpins are very loose on the wheels until the retaining nut is applied. Do you think this could be the cause of the problem?

 

Kenton, in reply to your question I now think both sides have tight spots.

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I have a problem with the nuts retaining the couplings rods tightening up on my current project. I think it is caused by the edges of the rods not being completely smooth and so catch the washer and then the nut and make it move in relation to the crankpin and tighten or loosen depending on the direction of travel.

 

I've decided to ignore it for now and hope a drop of loctite or similar fixes it when the kit is finished.

 

Regards,

David.

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Thanks again for all your help guys. Its very much appreciated. I think I’ve got the problem down to the crankpins as they are very loose in the wheels and don’t sit straight. I assembled the chassis using an alignment jig with both sides of rods on so surely this would eliminate in-correct axle alignment? Going back to the crankpins, what’s the best way to get these to sit straight in the wheels?

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I’ve also noticed when everything is loose (Nuts are not tight on the crankpins) it runs a lot better. However once everything is tightened, it really starts to jam up. Ive also noticed the crankpins are very loose on the wheels until the retaining nut is applied. Do you think this could be the cause of the problem?

 

If the rods are not just right then they will usually appear better when loose since the rods can then angle themselves to suit the crankpin centres. But it sounds like you have a more difficult problem. If the crankpins are loose in the wheels then you will not get anywhere until that is solved. Possible remedies depend on what the actual problem is. To be able to suggest anything there we need rather more info.

Make of wheels?

Make and size of crankpins?

How crankpins are fitted?

Are the crankpins bushed?

How does the nut cause them to tighten up?

Regards

Keith

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The wheels are standard Slater’s with 12BA screws acting as the Crankpins. On the outside I’m using the normal brass top hat bushes which goes through the rods secured by a washer and retaining nut. Unfortunately the holes in the wheels are slightly too big for the 12BA screw which has resulted in a lot of play. Instead of screwing into the wheel, they seem to slide right through. I’m considering using Loctite to secure the screw in place but I think getting them dead straight is going to be a problem.

 

If the rods are not just right then they will usually appear better when loose since the rods can then angle themselves to suit the crankpin centres. But it sounds like you have a more difficult problem. If the crankpins are loose in the wheels then you will not get anywhere until that is solved. Possible remedies depend on what the actual problem is. To be able to suggest anything there we need rather more info.

Make of wheels?

Make and size of crankpins?

How crankpins are fitted?

Are the crankpins bushed?

How does the nut cause them to tighten up?

Regards

Keith

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The wheels are standard Slater’s with 12BA screws acting as the Crankpins. On the outside I’m using the normal brass top hat bushes which goes through the rods secured by a washer and retaining nut. Unfortunately the holes in the wheels are slightly too big for the 12BA screw which has resulted in a lot of play. Instead of screwing into the wheel, they seem to slide right through. I’m considering using Loctite to secure the screw in place but I think getting them dead straight is going to be a problem.

MM

I would replace the Slaters crankpins for those from Derek Mundy which require a larger hole in the wheel 3/32" or 2.4mm.His address and phone No. are

 

Derek Mundy

2 Lon Eirlys

Prestatyn

LL19 9JZ

 

01745 852989

 

Or you could return the wheels to Slaters.

Hope this helps

Tim

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Tap out the crankpin holes 10BA and fit 10BA countersunk screws from the back. Tap out the bushes 10BA (or buy some from Gladiator) Using this method the crankpins are clamped to the wheels and solid. You won't get reliable running if the crankpins are slopping about.

Nick

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Tap out the crankpin holes 10BA and fit 10BA countersunk screws from the back. Tap out the bushes 10BA (or buy some from Gladiator) Using this method the crankpins are clamped to the wheels and solid. You won't get reliable running if the crankpins are slopping about.

Nick

 

 

 

I'd second Nicks method. I've been doing it this way for about 7or 8 years now and have had no probs. Just remember to countersink the screw head into the back of the wheel.

 

OzzyO.

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Tap out the crankpin holes 10BA and fit 10BA countersunk screws from the back. Tap out the bushes 10BA (or buy some from Gladiator) Using this method the crankpins are clamped to the wheels and solid. You won't get reliable running if the crankpins are slopping about.

Nick

 

I've done this too, but I find sometimes the bushes are not long enough to fit all the way through the rods. In this case, do you just screw them as far as they will go (without locking everything up) and secure them at that position? I had turned some tiny top hat bushes to fit between rod and wheel to make up the difference and stop the rod rubbing on the wheel, but making such small turnings is difficult.

 

Regards,

David.

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Hi David

Rather than turning a extra bush I just use a slice of brass tube for plain rods. Where a return crank is needed use two bushes with one reversed, and for

the leading crankpin when there is tight clearance just reverse the bush. My feeling is tho' that is the bush is too short the rod is to thick in many cases.

Nick

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Hi David

Rather than turning a extra bush I just use a slice of brass tube for plain rods. Where a return crank is needed use two bushes with one reversed, and for

the leading crankpin when there is tight clearance just reverse the bush. My feeling is tho' that is the bush is too short the rod is to thick in many cases.

Nick

 

Good idea re the brass tube, I'll try that.

 

I agree about the thickness of the rods. On my 4-4-0 I have only used two of the three laminations because the rods on the 0-6-0 look too thick.

 

Regards,

David.

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My thanks to everyone who has replied. Its been a great help.

 

Nick, Is there a special method/tool used to get the crank's dead straight when it comes to tapping out/screwing the screw threw the wheel?

 

Cheers

 

MM

 

Tap out the crankpin holes 10BA and fit 10BA countersunk screws from the back. Tap out the bushes 10BA (or buy some from Gladiator) Using this method the crankpins are clamped to the wheels and solid. You won't get reliable running if the crankpins are slopping about.

Nick

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Mk one eye ball works for me, or you can use a square to judge that you are running the tap in square. There is a tool you can get from the USA can't remember its name but cost lots of dosh, that will help. You can use it to do rivets as well.

 

Sorry bin to the MRC tonight and me heads trying to get over the new club rooms.

 

OzzyO.

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I do it by eye, but I'm not convinced all mine are straight :-\ With my 0-6-0 kit fold-up chassis it is hard to tell if it is the axles or the crankpins which are out of line but the right hand side needed a lot of work. With the 4-4-0 scratchbuilt chassis I drilled minimum sized holes in the rods and they went straight on with no binding and no slop.

 

You could make a sort of tall top-hat bearing with a hole sized to just clear the tap and rest that on the wheel to ensure the tap goes in straight. I've seen that idea used for other tapping operations. You'd have to ensure the tool was made accurately, and that the back of the wheel was very flat.

 

Regards,

David.

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