Diametral Pitch of Gears & Hobs

Hi all,

I'm currently trying to design some gears for an automotive transmission which involve working with a fixed imperial center distance of 2.8 inches. Mathematically the only "standard" DP value that I can use with this center distance is 10. This not only limits the ratios I can choose from, but it also limits the strength of the gears as a DP 10 tooth is quite small for an automotive transmission.

Looking at the maths, I can also use a 6.25, 7.5 and 8.75 DP gear, which essentially solves the issues above. My only problem with this is that I am unsure if these are standard DP values that a gear manufacturer may have a hob or a shaping cutter for. I'd rather not design gears for a low volume product which will require special hobs / shapers to be made for them!

I feel as if DP values at quarter inch increments between 6 and 10 DP can't be rare, especially in the automotive world. However I am finding it very difficult to find any reliable information anywhere on the net that can confirm this or explicitly deny this.

Looking at some performance transmission suppliers websites, they offer a large variety of ratios for use in the same transmission. Assuming a fixed center distance and working backwards using the tooth counts, different gear pairs give different non-standard but sensible DP values such as 7 and 5/6th DP and 8 and 1/6th DP.

Does anyone know if sensible non-integer DP values are used in the industry on a somewhat frequent basis if at all? Or point me in the direction of somewhere that discuss or proves this?

Thanks!

Virtually no gears in volume automotive transmissions are 'standard' gears. In order to get the desired ratios, centre distances, torque carrying capacity etc they are typically designed on a case by case basis. In addition, unless you are looking at racing gearboxes etc, they will be helical gears and of course you can play with these to get the ratios you desire with a fixed centre distance. The volume of manufacture is so high that the additional cost of special tooling is usually not a serious problem.

You need to assume HELICAL gearing. You can have the "DP" you want because its a function of center distance and tooth count - and the resulting helix angle

Handy facts in thumbnail. Note the desired center distance (18.75) is LARGER than the straight cut center distance which would be 18" for 4DP 48 and 96 tooth gears.

Way more than I'll ever know in such as Manual Of Gear Design Section Three Helical and Spiral Gears by Earle Buckingham

There are folks here who know way more about this than I, but if you google Profile shifted gears or similar terms you'll get a lot of information on how to adjust spur gears to match a center distance. Here's an example - Calculation of Gear Dimensions | KHK Gears
I've heard this described in different ways by gear people, such as putting a 10 tooth gear on a 11 tooth blank and modified pitch diameter, but profile shift will get you started. You can't make these with normal milling cutters you need a gear shaper or a hobber so you can generate them. An extreme example will have odd looking teeth. I've seen modified gears that will work with standard gears, such as in QC rear end change gears so only one gear needs to be changed to change a ratio, but generally both gears get changed. That's about all I can share.

Also it is very commen in automotive to change pressure angle to optimizes the rolling element of the gears instead of rubbing
That also reduces noise
The cheapest tooling IMHO you get on a Maag type of machine
You can wire EDM the tooling for it

Peter

Thanks for the replies guys. It is actually racing transmission I am looking at which is of course low volume, hence the concern about tooling! This also means that I would like to use spur gears.

Johnoder, That picture probably explains why back-calculating from some physical helical gears a few weeks back gave me very slightly different DP values for some gear pairs, which left me awfully confused!

Mud said:

There are folks here who know way more about this than I, but if you google Profile shifted gears or similar terms you'll get a lot of information on how to adjust spur gears to match a center distance. Here's an example - Calculation of Gear Dimensions | KHK Gears
I've heard this described in different ways by gear people, such as putting a 10 tooth gear on a 11 tooth blank and modified pitch diameter, but profile shift will get you started. You can't make these with normal milling cutters you need a gear shaper or a hobber so you can generate them. An extreme example will have odd looking teeth. I've seen modified gears that will work with standard gears, such as in QC rear end change gears so only one gear needs to be changed to change a ratio, but generally both gears get changed. That's about all I can share.

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Thanks alot for this! I have been reading up on this following your post. I believe it has set me down the right path in terms of what needs to be done, though it may be quite a time consuming process to end up with some gears, which is fine! I have already designed the gears for the transmission based on quarter inch interval DP values (as mentioned in the OP) with symmetrical profile shift on the pinion and gear, so it would still be good to know if hobs with quarter inch interval DPs is something a gear cutting firm might have on their shelves. If anyone knows?

Thanks for the replies guys. It is actually racing transmission I am looking at which is of course low volume, hence the concern about tooling! This also means that I would like to use spur gears.

Johnoder, that pic might explain why i was getting very slightly DP values when back-calculating from some physical helical gears!

Mud said:

There are folks here who know way more about this than I, but if you google Profile shifted gears or similar terms you'll get a lot of information on how to adjust spur gears to match a center distance. Here's an example - Calculation of Gear Dimensions | KHK Gears
I've heard this described in different ways by gear people, such as putting a 10 tooth gear on a 11 tooth blank and modified pitch diameter, but profile shift will get you started. You can't make these with normal milling cutters you need a gear shaper or a hobber so you can generate them. An extreme example will have odd looking teeth. I've seen modified gears that will work with standard gears, such as in QC rear end change gears so only one gear needs to be changed to change a ratio, but generally both gears get changed. That's about all I can share.

Click to expand...

Thanks for this, I have been looking in to this a bit more since your post. I think it may have pointed me in the right direction for what I need to do. Which fundamentally is using addendum modification to get the desired center distance. And id imagine basing the calcs on module for this rather than DP as I have been currently doing will be a better bet. I expect it'll take a bit of work to get to some good finished gears, which is fine.

I have already designed the gears for the transmission using quarter inch interval DP values (as mentioned in the OP) and symmetrical addendum modification for pinion and gear, so it would still be good to know if hobs of this sort of DP are something a gear manufacturer is likely to have sitting on their shelves!

AllTheGearNoIdea said:

... so it would still be good to know if hobs of this sort of DP are something a gear manufacturer is likely to have sitting on their shelves!

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Nope. Not even remotely likely, that I've ever seen. And I've spent over a decade and a half of making oddball, custom Pitch ( and Pressure Angle ) gears for gear trains/boxes. In the past, we always had to order them, custom, pay for the pleasure, and wait the 8 - 12 weeks for them to arrive.

It is very much one of the factors that drove me to learn how to make our own custom gear shaping cutters. ( Along with Skip Tooth/Skip Space configurations and Asymmetrical Tooth Forms )

You may want to hit EG up and see if he can help you source the cutters you'll need, on that side of the pond.

You might want to try Cannon engineering in Leeds they do gear cutting. The owner knows a bit about automotive gears as he's a top fuel bike racer.

You can make some adjustment using standard cutters, that's how backlash is developed for example, but when you change one parameter at least one other will change, like changing the Pd with a standard cutter changes the effective pressure angle. I think experimenting with an inexpensive gear simulation program like Gearotic could be illuminating. If you are building to handle a lot of power with compact parts you might need to get a real gear engineer involved. Please post what you learn, I'm very interested in this.

You're way the hell over your head. I get calls like this every week. Instead of blowing people off insist they buy and STUDY these two publications before calling me back. Then we can have an intelligent conversation of transmission gearing.
Design Practices: Passenger Car Automatic Transmissions
Dudley's Handbook of Practical Gear Design and Manufacture: Stephen P. Radzevich: 9781498753104: Amazon.com: Books

AllTheGearNoIdea said:

And id imagine basing the calcs on module for this rather than DP as I have been currently doing will be a better bet.

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Barf. Barf barf barf, excuse me while I run to the bathroom and upchuck. The module system sucks long-dead, half-rotted stinking maggot-infested donkey balls.

And I finally agree with ten fingers on something. Must be the Christmas season