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Another question about gear calculations!

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Adam Harris27/04/2019 12:38:04
451 forum posts
19 photos

I need to replace the handwheel carriage spur gears on a Hardinge KL-1 (almost identical to the US Hardinge HLV-H) which are a 66T and 26T meshing. (The HLV-H actually has 70T and 21T meshing but that is a whole other matter). Both are 22DP 20PA. I want to replace them with easily available Myford change gears 20DP 14.5PA (although I understand that the small Myford gears 21T-24T might actually be 20PA too), and need to know what combination of Myford gears would mesh with the same Centre to Centre distance as the original. Having searched the internet I have come across various formula which include information I cannot provide other than the DP, Number of teeth and PA. Is there a simple solution to this question of the best possible choice. Mesh accuracy does not have to be absolutely 100% perfect because we are only talking about carriage drive via the rack (manual or electric motor), not thread cutting which is done by the leadscrew. Any help much appreciated!

Edited By Adam Harris on 27/04/2019 12:38:30

John Haine27/04/2019 12:58:37
3022 forum posts
160 photos

Well, the pitch diameters are 3 inches and 26/22 = 1.182 inches; the sum is 4.182; and the centre spacing should be 2.091, or say 2.1 inches.You will need a pair of 20 DP gears withe same centre distance. So you need a pair of gears with N1 and N2 teeth, where N1/20 + N2/20 = 4.2 (say), or N1+N2 = 84. I don't know what the full list of Myford gears is but if you just look at the list you should easily be able to find a pair if it is possible. But there may not be a suitable pair.

As an alternative look at stock gears from engineering suppliers such as this:

They do for example 64/20 tooth or 60/24 tooth combinations.

Edited By John Haine on 27/04/2019 13:03:42

Adam Harris27/04/2019 13:08:32
451 forum posts
19 photos

Thanks for your quick response John. I saw many complicated formulas for centre distance but the simplest was (T +T)/(2x DP) = 2.09 for the original set up, and so changing DP to 20 means the T +T should equal 0.91 x (66+26) = 83.7 which approximates 84 as you say. Does that really mean that the centre distance for 60T + 24T will be exactly the same as 40T + 44T?? Is any choice of combination better (medium numbers, small and large number) better for meshing with the 0.3 error?

JasonB27/04/2019 13:28:36
17874 forum posts
1954 photos
1 articles

Don't think you will get spot on using off the shelf 20DP but you could over cut one or the other to get the correct PCD. 24/60 is 0.018" too large. But if you are going to cut them then you may as well cut to the correct 22DP.

Probably even cheaper to by off teh shelf MOD gears from one of the bearing suppliers if you convert the 4.182" to metric that gives you 106.22mm. MOD 1.25 gears in a 21/64T combination gives 106.25mm and as that is halved you may get away with it being 0.015mm over thats six tenths of a thou.

Adam Harris27/04/2019 13:34:47
451 forum posts
19 photos

Thanks Jason. I will check the prices of MOD 1.25 21T and 64T but I cannot imagine they will be nearly as cheap as Myford which are around £10 each

Adam Harris27/04/2019 13:35:43
451 forum posts
19 photos

on fleabay

JasonB27/04/2019 13:36:56
17874 forum posts
1954 photos
1 articles

I've used Beltingonline a few times for gears on engines and they are well made and reasonably priced. Being EN8 they will machine OK if you need to modify them, a lot of the ones on the bay are hardened (dark teeth) so need annealing first.


Edited By JasonB on 27/04/2019 13:39:13

Adam Harris27/04/2019 13:40:56
451 forum posts
19 photos

Wow Jason, yes those are not much more expensive and also better/stronger being 20PA! Also easier for me to machine the correct size bore. MANY THANKS

Edited By Adam Harris on 27/04/2019 13:41:18

Edited By Adam Harris on 27/04/2019 13:50:51

Edited By Adam Harris on 27/04/2019 13:54:23

Adam Harris27/04/2019 13:48:23
451 forum posts
19 photos

I will go the MOD 1.25 route 25T + 60T

Edited By Adam Harris on 27/04/2019 14:07:37

Edited By Adam Harris on 27/04/2019 14:10:55

Pete Rimmer27/04/2019 21:10:29
685 forum posts
49 photos


If you want original 22DP 20PA gears I can hob them for you. You'll have to furnish me with the relevant dimensions i.e bore, thickness etc.


Edited By Pete Rimmer on 27/04/2019 21:11:53

Adam Harris27/04/2019 22:00:19
451 forum posts
19 photos

Thanks Pete, I have private messaged you. Adam

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