Gearbox parts

[richiereynolds]

Hi folks, I've a question about an EMG30 gerahead motor I have which has broken a tooth on one of the gear cogs.

Does anyone know of anywhere that would sell these cogs? Preferably in the UK?
And, any ideas on how I'd identify the part I need? I've searched the web for EMG30 specs but none of them detail the cogs, is "Cog" even the correct term?

If it helps I could provide a picture of the part.

Thanks in advance!

[Soeren]

Hi,

Hi folks, I've a question about an EMG30 gerahead motor I have which has broken a tooth on one of the gear cogs.

Does anyone know of anywhere that would sell these cogs? Preferably in the UK?

I could be wrong, but I don't think you'll be able to find spares for this motor anywhere, but you might be able to use it with another gearbox (or without) for something else.

It is ridiculous expensive I know. Perhaps you could find a used one for cheap where the brushes have worn out or something similar and swap the gears.

[richiereynolds]

I don't think you'll be able to find spares for this motor anywhere

Having had a further look online, unfortunately, I think you're probably right.

Think I might just buy another, at least I'll have some spare parts then for the replacement or, as you say, maybe find a different use for the damaged one.

It's a shame but I have a working emg30 on a bot so need the second one for a matched pair, c'est la vie 

[dave239]

Hello

I don’t want to raise your hopes too much, but it is possible that one of the hobby/model engineering companies that attend the various model engineering exhibitions in the UK may stock the required gear (cog). Failing that, there is at least one UK manufacturer that produces gears in most types/sizes/materials and will sell ‘1 off items’ to the general public.

Gears can be very expensive, which is why I now machine my own limited range of sizes and machine my own gearboxes. However if it is a small plastic spur gear of a common size then you might find one for anything from £1 upwards.

This does assume that you can actually get the gear off the shaft without causing damage and then securely fix a replacement.

If you still want to try to obtain a replacement the next thing to do is identify the gear. I may be able to help you if you post 3 photos (in the highest resolution that this forum will allow):

1)   A plan view (camera looking directly onto the centre hole) - with the gear laid flat and alongside (flat) a steel rule (or ruler)
        - to get dimensions and tooth shape
2)   A side view (again with steel rule)
3)   A view at 45 deg approx. to get an overall idea of gear type (again with steel rule)
   
It would also be helpful to know:

a)   How easily the gear came off the shaft (if indeed it has)
b)   What type of attachment is used – if that is not obvious from the photos
c)   The material type (plastic/brass/steel etc)
d)   The diameter of the gear ideally measured with digital calipers (avoiding the area of the missing tooth !!)
e)   The diameter of the shaft on which it fits
   
If the gear can be identified then a rough price can be found from the gear manufacturer’s catalogue that I have by my desk. This will give you an idea of a ‘top’ price and then you can decide whether it is worth shopping around.  (I can give you some company names as necessary)

I would however stress that unless the gear has been removed easily then the whole replacement process could be difficult and may cause damage to the motor/gearbox).

I hope this helps

dave239

[richiereynolds]

Here ya go, afraid I'm not much of a photographer and I don't have calipers, though I am thinking of investing in a set so if you think there might be a chance of a replacement part I'd be willing to get more accurate measurements.

https://picasaweb.google.com/104835163932962237504/Cogs?authkey=Gv1sRgCMm0zb6an8WG5AE#   

It just slides off the shaft with no problems.
All the cogs look like brass (or at least metal) to me but the larger diameter part (the bit with the broken tooth) is a different colour to the rest, appears to be reddish and it's mounted around the metal inner section, i.e. the smaller section is like a shaft and it's mounted (permanently) onto it. I think it's some sort of hard plastic.
The diameter of the larger section is about 10.25mm
The diameter of the smaller section is about 5mm
The diameter of the shaft it slides onto is about 1.5mm
The best EMG30 spec I can see is here http://www.robot-electronics.co.uk/htm/emg30.htm

[dave239]

Hello again,

The gear in question is unfortunately a double spur gear.

I say unfortunately, as finding an “off the shelf” correct combination of gears in a double gear is very unlikely.

Although it is somewhat difficult to work out from the photos, I believe the two gears in question in the broken item may be as follows:

The larger gear - 25 tooth MOD 0.4, OD 10.8 mm, 3mm thick made from Tufnol
The smaller gear – 10 tooth MOD 0.4 splined shaft (OD 5.2 mm) – could be steel or brass (6mm long with 3mm protruding from the larger gear).

It appears that the smaller gear (splined shaft) has been pressed into the Tufnol and then deformed at the bottom end to fix it.

(If they are not metric gears then they might be 64 DP but my guess is MOD 0.4).

Tufnol could have been used for a number of reasons including cost, noise dampening, or as a weak link, to be the first point of failure in case of a fault.

I have checked the catalogues of 4 or 5 of the hobby companies and they do not stock this size.

Moving to the specialist gear companies again the double gear in this specific combination is not a standard part (similar sizes are £30 plus – so more expensive than a new motor).

You could consider joining two gears. I join my own brass gears.

You can get the larger 25 tooth gear in for example brass for about £10 or plastic for about £3. Tufnol is available but prices are not given in my catalogue. (- prices exclude p &p)

(I can’t find availability/prices for double gears of this particular size but).

The problem though is how to fix the 2 parts of different materials. One possibility might be to use a brass 25 tooth spur gear and drill out the centre and insert the spline shaft section from your existing gear (having cut away the Tufnol). You might then be able to glue or braze the parts but you would have to be very accurate with the assembly. The problem with this is that you don’t know why Tufnol was chosen (over brass or steel) so introducing a brass gear may not be a good idea anyway.

You could try the cheaper Delrin 25 tooth gear using the same principle but it might be difficult to glue and Delrin is not as strong as Tufnol.

A third possibility is to fix together 2 new brass gears (one 10 teeth with a boss, and one 25 tooth centre drilled to take the boss of the smaller gear). However as each part costs around £10 it’s probably more expensive than a new motor.

At least you know the specification and might get lucky, but sorry I couldn’t find you a catalogue part. I will keep a look out at the exhibitions – it’s just the sort of thing that you find in the oddment bins !!

dave239

[richiereynolds]

Actually Dave that's a great help, even if I can't get the part I've learnt loads about gears in the process, lot of very useful terminology there! Plus I know the sort of thing to look out for.

Cheers!