Baffled by spur gear

[Bluechip]

Hi Folks

I despair. I have a Delrin spur gear. 16T, mesured at 0.625 in. OD [ or very close, tooth on side, space t'other ].

Using the traditional 'DP = (N+2) / OD' i get 28.8 DP.

This Delrin gear meshes perfectly with a steel gear 32DP 48T. [ marked on it, steel gear works out with formula ].

I seem to remember, or half remember, that gears with a small number of teeth can be 'corrected'. The formula then gives a wrong answer.

I've groped around my books, and the www. cannot find a reference to it.

Am I mistaken ? It has a 5/32 in. bore, so I reckon its not Mod. [ Doesn't work out to a sensible Mod either ]

Any ideas welcome.

Dave

 

[mklotz]

Are you sure you didn't miscount the number of teeth?

Eighteen teeth would yield 32 DP.

20/0.625 = 32

 

[Maryak]

Dave,

First thought maybe circular pitch ???


Regards
Bob

 

[andrewh]

Its been a long while since I wrestled with gears, but 16T isn't a small number of teeth!

Its the tiny sprocket gears, 12T or less that require modification so that that the "leaving and entering" teeth have enough room to do that

P'raps your delrin gear is made to module standards?
andrew

 

[Maryak]

Dave,

Second thought and Machinerys Handbook :

I think it's an English Module Gear as near as I can make out Pitch Dia would be around 0.504"

0.504/16 = 0.0315 = 1/32 = 32DP

Regards
Bob

 

[Bluechip]

Hi Folks

Thanks for replies, I think a friend has come up with the answer.

Marv. - Definitely 16T.

Bob. - CP is a mystery to me, with an odd gear , only thing I can measure are OD & No. of teeth.

Andrew - It did not seem to make sense with Mod, Equation I had was Mod = 25.4 / DP, but I had no confidence in my DP, so no further.

I meshed the 32DP 48T with an identical gear, also the Delrin gear. Using a hand lens the Delrin gear is somewhat different in tooth form. The apparent 'perfect' mesh ain't !! B*ll*cks!
The friend suggested these gears are 'specials', he has known Delrin or similar gears to be used on low speed, low torque drives as zero backlash components. They are run in slightly too tight mesh. If you measure them, they make no sense because the tooth form may well be modified, and may be selected to fit from a kit of gears of varying sizes. May be different pressure angle ? He also mentioned 'stub gears', something like 8DP cut to 10 DP depth. Who knows. He suggests I dump it. Have done. Sorted ;D

Thanks again.

Dave

 

[steamer]

They may be stub profile, to make the tooth short and therefor stronger. That may be throwing your calculation off. Measure the full depth of the tooth and compare it to other one and check to see if it is shorter in height

That may explain it.

Dave

 

[Bluechip]

Hi Steamer,

Can't check it now Dave, despatched to the Bin Fairies.

I had a box about 15" cube of 'plastic' gears salvaged from office printers etc. Decided to audit the contents and trash the oddments. Most measured giving answers like 31.96 DP or 29.88 DP which is acceptable, calyping ain't all that precise. Still a good quantity of useable bits left. Don't know if I'll ever use the things, had them about 25 Yrs.

My friend, (another Dave, , we need to start using Regnal numbers, I think), was not surprised with the curious results, according to him, for pure geometric accuracy, we should have a gear cutter for every No. of teeth in every DP or whatever. But engineering limits accept a compromise, compromises are compromised, and hence my strange gear.

Well, He's a MIMechE, and I do electronic stuff. There's no way I can argue !

Dave (another)

 

[steamer]

Your friend is right. There is other tooth forms for differant reasons too.

Good luck,

Dave