Finding a replacement gear

[Chuck in E. TN]

I need to help an acquaintance find a replacement gear for her fathers lift chair. She describes the gear as plastic, 2 5/16” diameter, 3/8” thick, and 55 teeth. The bore is ¾”. I have tried Stock Drive Products with no help.
She can buy an entire drive assembly replacement for the chair but the cost is prohibitive.
Is there an easy way to enter these specs to find a gear, or am I missing an important descriptor like diametric pitch? Any help in finding a replacement would be greatly appreciated.
Other than the description above, I do have a couple of pictures of the broken gear.

Chuck in E. TN

[Bluechip]

Hi Chuck

IIRC

 DP = ( N+2 ) / OD            where N = # teeth.

Using that I get something like 24.6 DP which makes no sense, but...

Allowing for small  errors in measurement, it MAY be MOD 1 ???
It's hard to measure the OD of some gears as you are on a tooth tip one side and a gap on the other ..
and they may not be exact to 'theoretical dimensions'.

'cos MOD 1 is 25.4 DP ...

err .. I think ...

[BillTodd]

That's very close to a 1 module gear size  (check the dims with a metric vernier).

Given that it is a person lifting device you may want to check if there are any insurance or safety issues with a DIY replacement.

Bill

[andyf]

Chuck, I have a 55T Mod 1 gear. Its OD is 56.8mm (should be N+2 = 57mm, but it's Chinese). 57mm is 2.244" , which is quite a bit less than your 2.3125", though you are relying on someone else's measurements, probably done with a rule.

If you have a mini-lathe, its changewheels will be Mod 1. Can you try one of them against the broken gear to see how it meshes?

Andy

[Chuck in E. TN]

I have contacted the person to verify tooth count and measurements. I do not have the gear at hand but may end up going to get it.
Here's the pictures she sent:





By my count on the picture it is 55 tooth, but can't verify measurements from the picture.
Thanks for your help.

Chuck in E. TN

[Chuck in E. TN]

Well, they brought me the broken gear tonight. It has (had) 55 teeth, 2.345 dia, .377 thick with a .750 bore. It meshes nicely with the change gears from my Micromark 7x14 lathe. Thus, I would guess its metric Mod1.
The person needing the gear showed me the webpage for a replacement set of 2 gears for the lift chair as a cost of over $90! Robbery! There has to be somewhere to get a suitable replacement for much less than that.
I’ve checked Boston Gear and Stock Drive Products and they don’t list anything close. Any ideas?
This is for a lift chair for a 83 year young Senior Citizen.

Chuck in E. TN

[Fergus OMore]

             If you will settle for cast iron and a slightly different cog, the 9x20 lathe gears should be OK.
It would need boring out but if you want a cheap solution, this might be it

[andyf]

Hi Chuck,

There's a steel 55T at Technobots (who will export) here in the UK for £11.53 (about $18, and being outside the EU, you won't have to pay the tax) plus shipping:
http://www.technobotsonline.com/steel-spur-gear-mod-1.0-55-tooth.html

They will be the same bore etc as some 60Ts I bought from them. One of those, before I started hacking it about, is shown in the first pic here http://andysmachines.weebly.com/new-banjo-dedicated-to-fine-feed.html (enlargeable version at the foot of the page). The bore is now 1/2", but I think they came as 10 or 12mm. The teeth were 15mm (about 0.6") across, and as you will see there is a big boss on one side, which gives somewhere to hold them in the chuck for boring. I then drilled/tapped a hole in the end of some square bar so I could bolt the gear to it, then held the bar in my bandsaw and sliced 8mm thickness (plus a bit to allow for subsequent facing) off from the bossless side. Subsequent deburring of 60 teeth on each side took ages.

Andy