Romford 20:1 Worm and gear

[Giles]

Can any kind soul tell me where I can buy the above 20:1 (1/8" axle) Gear and (3/32") worm, by any chance?

I'm aware that one can order direct from Markits, but does anyone know of any shops or suppliers that might have them?

 

 

Am in the process of building a RC 0-14 Bagnall. and I'm very limited for choice as to what bits are actually appropriate.

 

Thanks!

 

GIles

[Joseph_Pestell]

Very few shops these days that stock useful bits like this.

 

Branchlines may be able to help or, nearer to you, Roxey Mouldings.

[Graham456]

South Eastern Finecast do usually stock old size Romford gears and will take them to shows if asked but you would have to mail order from them normally so you might as well go direct to markits!

Shies at the loss of mainly trains,,,,,

[hayfield]

South Eastern Finecast do usually stock old size Romford gears and will take them to shows if asked but you would have to mail order from them normally so you might as well go direct to markits!

Shies at the loss of mainly trains,,,,,

 

 

I would try phoning Dave Ellis at Southeastern Finecast or Dave at Roxey Mouldinga first, as he/they will take payment over the phone and send the item out quickly. Markits work mainly via email, rarely answers the phone and the order processing in the past takes a little while

[Giles]

Thank you Gents - I will try some of these

[bertiedog]

I think Scale link stock these gears, I'll check

[bertiedog]

Markits direct seem the best, as Scalelink do not do that combination.

Stephen

[Giles]

Thank you......

 

However, Holts have come up trumps - exactly the right ones and instant delivery!

 

This combination of a 5:1 gearmotor driving through a 20:1 Worm and gear gives me a very compact 100:1, with a low power consumption motor (in this instance stall on 0.7A @ 6v)

 

Thanks to all!

[Giles]

I have shortened the worm, and bored it out to 3mm, and this is the result. The motor gearbox and worm is 36mm long, 10 x 12mm, and gives 100:1 total reduction.

 

[t-b-g]

I have shortened the worm, and bored it out to 3mm, and this is the result. The motor gearbox and worm is 36mm long, 10 x 12mm, and gives 100:1 total reduction.

 

 

That looks like a superb drive arrangement. Can you tell us about the 5:1 motor drive please? Is it generally available to the rest of us or some sort of home made/special order creation? I can think of a few applications where I could use something like that myself.

[Giles]

It's a Polulu 2376 5:1, MP (medium power) 6v micro gearmotor. This particular one came off EBay, but it won't be unique. They certainly aren't as common as the types usually seen though..... I had to search for them!

[RexAshton]

Found these here

 

A search on the manufacturers website also shows the same motor for 12v.

[bertiedog]

Be a bit careful in selecting these micro geared motors, most come from China and HongKong, and are top quality units, but it is the descriptions that cause issues.

 

They are widely listed on Ebay from a few pounds up wards, but the price and quality are un-related, as these are basically surplus items being sold off.

 

They were designed originally for use in Auto focusing lenses and cameras, in 6 volt and sometimes 12 volt versions. This market is now disappearing as stepper motors are now used.

 

Millions of these have been made and found other uses. Model cars, robotics and low speed drives like turntables.

 

The sellers usually rate them as a quoted Revs Per Minute at the rated max voltage, say a 30 RPM at 6 volts, what they often do not state is the gear ratio of the gear box, which can be up to 1000:1 on some very slow 5 RPM models.

 

It is a bit unusual to find one with only 5:1 gear ratio fitted, this makes a good match to the 20:1 of the Romford Markit gears.

To take an example a motor rated at 1000 RPM would usually have a gear box of about 30:1 and give a 600:1 drive overall.

Most Model railway engines do not need 600:1 so the 5:1 type would be far better.

 

But most Ebay sellers do not show 5:1 in the listings, they start at about 30:1, selling them as 800 to 1000RPM.

 

If you need to get a 5:1 then go to the specialist site listed in the previous postings, or do a net search for N20 type motors.

 

You may strike lucky on ebay and find a low ratio one, it will cost less than the suppliers off ebay.

 

Hope these notes help,

Stephen

[bertiedog]

Also do not forget the worm will have to be re-bored in a lathe to fit the 3mm output shaft, although if you have the space a joint can be added to couple the 3mm shaft to say a 1.5mm bore worm, in a separate gearbox. This would not need a lathe. The coupling can be silicone tubing.

 

With a lathe you can easily remove the final shaft and re-turn it to say, 2mm, a common worm size.

 

A small point about re-drilling the worm to 3mm, to do this accurately the worm should not be gripped over the spiral teeth, the top surface will not be concentric with the bore, unless done by an unusually accurate machinist.

 

The way to grip the worm is to wrap brass wire around the spiral, and then grip the wire by a smooth collet chuck, not a 3 jaw, or simply make a split collet to suit your chuck. The wire can be held in by superglue, which is removed with heat afterwards.

 

The accuracy will be 100% with this method as the wire holds the spiral true to the machining.

 

I have seen people grip the worm direct in a three jaw, it holds, but damages the spiral top surfaces, causing massive wear to the gear teeth. You can slip the worm into a collet but as I said the outer diameter may not be true to the teeth.

 

Stephen

[Giles]

Found these here

 

A search on the manufacturers website also shows the same motor for 12v.

The one shown is a 50:1, rather than a 5:1 - however, here is a link on the same site to a 5:1 medium power version

 

https://www.pololu.com/product/2376

[Giles]

Excellent point -

(I made a split colled from tube to hold my worm - very easy, and i keep such things for the next time!)

[bertiedog]

My point about wire wrapping a worm is due to long experience in the 60/70/80/'s of Romfords worms! They were frankly awful, and the spiral track was not concentric with the bore, so most commercial work I did was re-boring the hole and sleeving back to 3/32nd.

 

The crown of the spiral worm tooth was often not true to the teeth and they left burrs there as well.

 

Oddly the 20:1 gear set was the most accurate and the spiral cut was done to machine the crown at the same time, the correct way.

But 40 and 60 were a joke, the machining was done in stages and lurched from one datum to the next, often ending up without any machined surface true to any other.

 

It smacks of an Auto Brittain lathe set up by a shop manager, and then any handy workshop bod being given a couple of days work till the tools needed sharpening!

 

None of this applies to the Markits versions, they are probably made on a CNC machining centre, where the work remains on datum throughout, all surfaces to the program limits.

 

Wires are also used to support spur gears in collet chucks for all the same reasons, gives reasonable accuracy.

 

Stephen

[bertiedog]

Thinking back over why the 20:1 gear was better made, I bet they used a single cutter from a Clarkson auto threader, and cut by chasing the thread along the work, using the multiple teeth in contact to give a better finish. I suspect the TPI figure matches the pitch of 20 to one, but the other sizes do not match exactly, so a single point cutter was used for higher ratios.

 

In instrument machining, that I did a lot, I often used a Clarkson cutter to chase threads, or used Swiss chasers, as chisel like tool that has a thread form on it's end, used to hone the thread to exact fits. But where the TPI matched I could use the Clarkson jaw clamped in the tool holder and chase a thread in one go, the left hand edge teeth doing the cutting and the right hand end chasing the thread to a polished appearance.

 

Does not always work as the tooth form may not match correctly, straight versus involute form etc. 

[t-b-g]

Some very interesting points and tips there and thanks for posting them. I am quite lucky in that I have a couple of good friends who are a very handy with machine tools. One did a tool making apprenticeship and the other was involved with such things all his working life. I always ring one up for "advice" on such matters, as I have a lathe but very little experience.

 

The conversation usually goes "How should I set about doing this?" and the answer is usually "You send it to me and I do it". I would like to learn myself but I would use the knowledge so infrequently that I would probably forget what I had learned between sessions.

 

With the demise of Mashima, these alternatives are worth investigating although things like power, torque and speed are always going to be a bit of an unknown variable to those of us who don't know what "good numbers" for these things are.