20 degrees vs 14.5 degrees pressure angle for gears

Hi,

I'm looking to make a set of spur gears for my 3" Burrell. The drawings show these as 10 DP with a 20 degree pressure angle and between 17 and 70 teeth, however I'm struggling to find an affordable set of cutters with a 20 degree pressure angle (£100+ a cutter is not affordable). RDG Tools do an affordable set of 10 DP gear cutters at 14.5 degree pressure angle. How much of a difference would the two pressure angles make (understanding that the whole train needs the same DP and pressure angle) or can anyone recommend a supplier who can supply 10 DP gear cutters with 20 degree pressure angle.

Many thanks,

Martin

It won't make much difference and will be closer to the original gear profile as a bonus and works better if you have a sprung back axle.

Martin,

If it helps to visualise what Jason has proposed, I attach a portion of a drawing showing a 70 tooth 20° PA 10 DP gear ( the lower gear portion in black ) with a gear portion of the same specification, but 14.5° PA ( in red ) superimposed, meshing with a portion of a 17 tooth 14.5° PA 10 DP gear ( above ).

It was generated using GearDXF program and manipulated in QCAD Professional.

John

The 20 degree PA teeth are more "triangular" (less parallel) than the 14.5 and one result is a larger radial force pushing the gears apart when a given torque is applied. The teeth are also slightly more robust.

Murray

Martin,

There is an alternative and that might be to cut the gears as a metric set, 2.5 Mod and 20 degree PA.

The cutters might be more competitively priced for those and your build notes would record the change from plans for the benefit of someone further down the line.

I'll now retreat to the defended bunker

Brian

Martin,

Perhaps I should add that if you go down the metric route you will need to make adjustments to reduce the centre to centre distances between the shafts accordingly from those shown on the drawing. They are not large, but would be important.

Brian

You can of course choose any module size you care to define, not just nice round numbers. You wouldn't be able to buy an off the shelf cutter for that exact size perhaps but for the purposes of making a functioning scale model, you could surely use the nearest "standard" size which may be only marginally different. That way you wouldn't need to mess with the centre distances.

To calculate your custom module size, you'd divide the reference diameter by the number of teeth. If there is a std module size close to that, you can take a view on whether it is close enough not to worry about.

As an example, I recently specified some gears in a mixture of mod sizes including 0.98. As the mould was going to be specially made anyway, it was no skin off anyone's teeth if they didn't all come out as exact integers.

Murray

Edited By Muzzer on 15/01/2018 12:07:45

You can make a gear abnormal or subnormal, ie the PCD is slightly more or less than normal for a given module or diametral pitch. I often used gears like this were gear centres were fixed for whatever reason. It does give a weaker root to the tooth when subnormal. Ideally you don't want to go more than half a tooth less or more, but this depends on ratio required.

In practice pressure angle only needs to be matched for each pair of gears, but it will look better if the whole train share the same pressure angle.

In practice older equipment/machines typically had 14.5 degree gears so Jason is probably correct that they will actually be more correct in appearance.

Pressure angle does not change any gear calculations, except it affects when you need to make allowances for undercutting when making pinions with small numbers of teeth (14.5 degree gears get undercut at 32 teeth, 20 degree at 17 teeth).

I'd guess 20 DP is specced because your pinion is 17 teeth. If you use a 14.5 degree cutter you can get around this by making the pinion slightly oversize (typically by about 1/4 of a tooth depth) and either increasing the gear spacing or making the mating wheel undersize by the same amount and accepting 'squarer' teeth.

This works better with hobbed than form cut gears.

Neil

Many thanks all for the advice and support. Neil, I'm slightly confused by your comment about undercutting, if the tooth form requires undercutting, I'm guessing that can't be done by a disc shaped gear cutter?

No, it requires a hobbed or generated gear to make an undercut, but if you use a pitch circle shift you can cut a usable gear with a form cutter.

Neil

Martin,

A little exploration of this superb utility should prove enlightening: **LINK**

http://hessmer.org/gears/InvoluteSpurGearBuilder.html

MichaelG.

That's generally what I use to create STLs for 3D printing.

Hi Martin, just quick question did you go with the

14.5 deg PA ? im in the same boat thinking of building

the same model and cost of the cutters.

RDG seem a cost effective solution but only if the 14.5

worked out.

Stephen.

Thanks to that diagram, I now know what the difference is.

Hi Stephen, I went for the specified 20 deg PA in the end. Steve Holder at sales@homeandworkshop.co.uk was able to supply the cutters I needed second hand at a reasonable price.

Here are some of the resulting gears:

Hi Stephen, I went for the specified 20 deg PA in the end. Steve Holder at sales@homeandworkshop.co.uk was able to supply the cutters I needed second hand at a reasonable price. Here are some of the resulting gears:

Fantastic looking job Martin, I feel reading the entire thread you did the right thing, I will try to follow the same.

Keep up the good work, and keep posting I'm sure it helps lots of people, not just me.

Just ordered the first few bits from Bridport so now in committed.

Regards Stephen.

Cutters Obtained!

Just an update I ended up going to the Midlands model engineering show and dropped lucky with the same Steve Holder. (Home and workshop)

I bought 3 of the cutters needed to start cutting gears for the engine at a reasonable price and all-important 20deg cutters. Actually started machining the smokebox tube and a few castings purchased my other finished engine can be seen to the right hiding 1" Minnie. The new smokebox being 3 times larger.

Edited By Stephen Meredith on 01/11/2019 15:57:17

One final question martin.

What did you use for the rear axle,

Is obtaining the required length in silver steel the way

Or is a simple MS bar polished up enough to run smooth

In the bearings. I can imagine a ground bar being pricey especially as its longer than 13" the standard for silver steel.

Any help appreciated from anyone.

As per the drawing I'm using 1-5/8" diameter EN8 for the rear axle on my 4" Burrell. The finish of the bar as drawn is fine as far as I'm concerned. Of course I'll remove any dings and burrs, but as drawn it's probably better than the original. I dispensed with the bronze bearings in the final drive tube as well. So it's steel on cast iron.

Andrew