Specifying a Spur Gear

[Pud]

Hi folks,

It's been a long, long time since gear days...

Am I correct in assuming that a gear can be specified by just the number of teeth and the modulus?

i.e. The two parameters then determine the o.d., pcd, etc? (I know the tolerances are another thing altogether!)

Cheers

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Occupant]

In addition: pressure angle, profile shift (if any) and I would also say "involute gear"

 

[Pud]

Many thanks Occupant, pressure angle indeed. Not sure what profile shift is though? Is this where, say, the addendum is modified from standard geometry (for instance)? And yes, involute! Good point!

Cheers

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[gearcutter]

As a manufacturer; for a gear without heat treatment or profile corrections such as crowning, tip relief, tip chamfering, root relief etc..............this would be all that I'd need from you.

Assuming we're talking about a spur gear that's not being manufactured to a standard;

Pitch - DP, CP or Module

Pressure angle

Number of teeth

Major diameter

Minor diameter

BTL (Base tangent length) over a specified number of teeth,
or MOP (measurement over pins/balls)make sure to specify pin or ball size,
or CTT (circular tooth thickness at the pitch diameter)
Ensure that you have included the backlash allowance in these measurements.......don't leave it to the manufacturer to decide on the amount backlash.

It's not necessary to quote addendum or dedendum corrections if all the above info is provided.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

Pud,

gearcutter's comment "....don't leave it to the manufacturer to decide on the amount backlash" should apply to every aspect of your gear engineering documentation. Unless your engineering/purchasing documentation is detailed and explicit then you are leaving design/manufacturing decisions up to your supplier. Gear companies are in business to make a profit and when given a choice they will always select the lowest-cost option, which will usually not be what you really wanted.

If you are not capable of creating the engineering documentation yourself try contacting your gear supplier (if there is one). Reputable gear houses are usually very happy to provide guidance to their customers regarding drawing requirements (backlash, tolerances, gear quality, geometry modifications, materials, etc) since this helps ensure the customer gets a product they are happy with and the supplier does not have cost/schedule overruns from rework or design changes.

It would also be money well spent to hire a gear design consultant to produce your engineering documents and serve as liaison with your supplier. Your gear sounds like it's very basic, so a competent gear designer should be able to do the job in just a few hours. Think of it as cheap insurance.

Good luck with your project.
Terry

 

[Pud]

Hi folks,

The saga continues...

We have been asked to mould the worm and spur gear in the picture (attached). To do this we (or rather our toolmaker) needs a 3d model of the item. We have contracted out this design to a company who said they could do it. On their first attempt, the spur gear did not "look" like an involute - more like spikey things. Apparently they sectioned it and traced the profile off a projector.

Now I have a drawing of the mating gear and the specs on the drawing are as follows:

Module: 0.7
No. Teeth: 9
O.D.: 8.25
Tooth thickness:3.36
3 tooth length: 8.03

Query is: Does/should a designer need any more information to draw the gear which is on the end of the worm - given that we now know the mod is 0.7 and there a to be 20 teeth on it? (I am going to assume it's a 20 degree pressure angle as I have no means or know how of checking that)

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Occupant]

I think so. Your pinion should have an O.D. of 9*0.7+2*0.7=7.70, yours is 8.25. Therefore it appears you have a profile shift of x=0.39 - unless they just made it bigger for some reason. But how can the tooth thickness be 3.36 when it is ~8.03 over three teeth including the gap?

 

[gearcutter]

Pud,

Based on the information that you have provided; your contractor has know idea what they are doing.
What's the job worth to you?
Have you considered engaging the services of a gear designer?

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

*no idea*

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Pud]

Many thanks for the help so far guys - getting there I think.

Gearcutter: We did ask our local University, who have an all singin' and dancin' gear research department, to do the job. Problem was their quote would have paid for the mould tool and half a ton of carbon fibre filled material at around 15GBP per kilo!!

Anyway, I have obtained the metal mating gear, and the dimensions as best I could measure them are as follows:

Nine teeth still
Measurement over 1.5 pins = 8.98
Single tooth max width = 1.35/1.36
Two teeth width = 3.37/3.39
Three tooth width = 5.46/5.47

Fuzzy picture attached.

Occupant: the previous "three teeth" figure I have realised today is the measurement over three teeth on the diameter. The gear measures 8.05 diametrically over three teeth. Hope that makes sense. (Drawing is annotated in English translation from Chinese).

Many thanks for your patience and helps...

Cheers

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[gearcutter]

You don't need to go to a research facility to find a consultant to do your design work.
The UK has many qualified consultants that would be able to help you out.
Several are listed on the AGMA website.
What would the cost of remaking your dyes be should you get it wrong the first time?
Designing tooling for plastic moulding is not quite a simple as just designing the gear geometry. There are many other factors to consider such as uneven shrinkage.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

Based on the measurements that you have provided;

The calculated base pitch is 2.09mm.
Actual base pitch for 0.7module x 20deg PA is 2.06mm.

As there is a mismatch in the results; there is an error in the information that you have provided.............with either the pitch or pressure angle.

On larger pitches; the small amount of error is usually not a problem. But on these very fine pitches; the error suggests that there could be subtle alterations from a 'standard' design. This is not at all unusual for plastic gearing.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

There's also been a tip modification applied to the data you have posted.
The first image shows a profile with a standard diameter, note the tooth's tip width.
The second image shows the profile as per your data, note the increase in tip width.


Standard OD by ronvol, on Flickr



Topped OD by ronvol, on Flickr




Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Pud]

Hi Ron,

Many thanks for your time and information.

I have attached a photo (as best I can do with phone) showing the tooth form of the 20t gear on the worm. It appears to be close to your first image. I suspect that the small difference of .03 (.0012") in base pitch is a combination of production and measurement errors, and as you know, plastics are not the most stable of materials at the best of times! I know mould tool design for plastic gears is a bit of a black art, especially in stuff like acetal with it's large shrinkage rate and variability with thickness, but this is to be made of much lower shrinkage carbon fibre filled high temp material.

I feel pretty confident that we (that is, you guys!) have reached a solution with enough confidence to have a rapid prototype made and see if it fits!

I will keep you posted.

Cheers

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Pud]

Oops: Photo!

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[gearcutter]

Pud,

Sorry but my pictures don't look anything like what you've posted.
My images are of the pinion, not the gear.
Notice the near undercut profile below the dedendum. This is one of the main reasons for the addendum correction; to help get rid of the undercut when using numbers below 18 teeth when dealing with 20deg PA profiles.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

gearcutter is correct. You should not have any undercutting of the flank profile with a molded gear, since the part will not release from the mold. If you require a small number of teeth on your pinion, then you should consider using a higher pressure angle value to prevent undercutting.

 

[Pud]

Hi Folks,
Minor update to saga. Have had the cad done and a couple of prototypes made. Customer has tried them and they are "over the moon" with them. Production tooling ordered and underway.
Pic of RP attached fyi.

Many thanks again - you've all been a great help.

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[tbuelna]

Pud,

Thanks for posting an update on your project. It's nice to know that the participants on this forum were helpful in resolving your engineering problem.

Regards,
Terry

 

[Pud]

Final conclusion: Customer happy and in production.

Pic shows parts on runner - makes one shot every 20 or so seconds. Material is PPA +30% CF. The tool has 4 slides on each cavity to make the worm as it becomes undercut with two.

Cheers

Harry

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.