timing belt gears

[firebird]

Hi

Can somebody help me out with a bit of maths (not my strong point)

I need to cut a couple of gears to suit a toothed belt I have. The belt has a tooth pitch of 5mm. Lets say i want to cut a gear with 30 teeth. Is this the correct formula?

Number of teeth = 30
Pitch = 5mm

30 x 5 = 150
150 ÷ 3.1415927 = 47.746482
Diameter of pulley blank = 47.746

Cheers

Rich

 

[agmachado]

Hi Rich,

I'm not sure, but I will confirm in the CAD... Soon I post of the result !

Cheers for you too,

Alexandre

 

[shred]

Timing belt pulleys (they look a bit like gears but aren't) are likely to require a somewhat different formula since the tooth shape is much different.

My Machinery's Handbook doesn't seem to have that topic, but I'd hit SDP-SI.com and if they make one like you want, they'll have full specs on it, like say this one: https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=8499&GroupID=218 (to pick a random 5mm belt one with 30 grooves in it)

 

[Jasonb]

The size you have come up with is the PCD, the actual dia of the pully over the teeth is less, can't tell you how to calculate by how much but HPC give the OD as 46.95mm

http://www.hpcgears.com/newpdf/timing_pulleys_5.0mod.pdf

http://www.hpcgears.com/newpdf/technical_timing.pdf

J

 

[firebird]

Hi

Thanks for the replies.

This is where I got the formula from. I copied and printed this section but I can't find the web page I got it from.

There is usually ample tooth space on commercially made timing pulleys, that I don't think that that is critical. Smooth corners with nice little radii would be good, of course, to prevent chewing the belts up.

The really critical dimension of a timing pulley is its OD. Actually, its OD is most commonly referenced as the pitch diameter, because that is where the belt's true pitch circle is created. The belts are manufactured with very exact and accurate tooth spacing. If this pitch diameter of the pulley is not exactly correct, then the belt is going to tend to 'crowd the teeth' on the pulley, and cause wear. There is a fair amount of frictional grip of the belt on the OD of the pulley, and the belt does not readily fall back into the gaps created for the belt teeth. So, "hack out" the tooth space however, but make sure the OD of the pulley is an exact multiple of the belt pitch.

eg.: 3/8 belt pitch, 10 tooth, OD of pulley is (10 * .375) / 3.1415927

I've machined a few timing pulleys both with form cutters and with slitting saws. I typically would gang a pair of slitting saws far enough apart to straddle two teeth on the blank, as this creates a bit of a natural angle on each side of the tooth face. That leaves the work of filing or sanding a bit of a fillet radius on the top corner of the tooth before the pulley is put into service.


Cheers

Rich

 

[jim216]

Rich,

My first and only foray into timing belts was converting my X3 mill to belt drive. I used a 5mm x 300mm HTD belt. I started will 3mm drill bits but wound up using an 1/8" end mill. In spite of my lack of skills it works well.

I used this calculator

http://www.sdp-si.com/Cd/default.htm




More pics
http://profile.imageshack.us/user/stovebolt

Uploaded with ImageShack.us

 

[bob ward]

These guys http://www.hobbyparts.com.au sell lengths of toothed pulley extrusion for a variety of timing belt profiles and tooth counts. More to the point for this thread is that they also show the OD of the various extrusions, eliminating the need for mathematics.

 

[firebird]

Hi

Thanks Jim and Bob, very useful

Cheers

Rich

 

[Simon0362]

And for use after you have created those pulleys.....

http://www.davall.co.uk/downloadlibrary/BELT CENTRES.xls

which is a very lightweight Excel programme to determine the centre distances - or can be used to determine the length of the timing belt if the centres are known.
Regards,
Simon

 

[firebird]

Hi Simon

Thankyou, also very useful

Cheers

Rich

 

[firebird]

Hi

I have pursued this a little further. Let me show you what I have done so far. I would appreciate any comments/help/advice.

I have a couple of belts, both are from the X3 milling machine. The earlier type (lower left in the photo) is a thinner 1/4 inch pitch square tooth belt and the later type (upper right in the photo) is a heavier 5mm pitch belt. I decided to have a go at the 5mm pitch.



Using this link very kindly supplied by Bob Ward

http://www.hobbyparts.com.au/store/item/pg5024x200alu/pulleystiming5000mmpitchgtstock/

I came up with the following info for a 24 tooth pulley



Category: Pulleys - Timing - 5.000mm Pitch - GT - STOCK
Grooves (No.):: 24
Min. Usable Length (L) (mm):: 200.00 (7.8740 inch)
Pitch Diameter (PD) (mm):: 38.20 (1.5039 inch)
Outside Diameter (OD) (mm):: 37.05 (1.4587 inch)
Material:: Aluminum Alloy
Shank Diameter (D1) (mm):: 12.70 (0.5000 inch)
Shank Length (Sl) (mm):: 19.00 (0.7480 inch)

Turn a piece of ally bar to 37.05mm. I am going to need 2 gears of this size so I have turned enough to get them plus enough for a spare or 2.



With the rotary table set up in the mill I set a 2.5mm slitting saw to centre height.



Then aligned the tail stock.



The work now mounted between rotary table and tail stock



Cutting in progress. I'm cutting to a depth of 2mm and advancing the rotary table 15° at a time to give 24 teeth.



All the teeth cut.



It looks pretty good but the belt doesn't fit too well.



On closer examination the belt has rounded teeth so maybe I need to round over the top of the teeth???
Have I started with the correct diameter of bar???

Any ideas

Cheers

Rich

 

[Admiral_dk]

I'm no expert on this subject, but you used a 2.5 mm slitting saw and it has parallel sides. Looking at the "teeth" you made, they are trapezoid in shape and the "wrong way around". This suggest to me that your cutter shouldn't have parallel sides, but should be a trapezoid with rounded corners, so to speak and since the belt has the shape it has, I'll guess that the angle should be a little bigger that you would think.

It's also obvious to me that the correct cutter will only be totally correct for one diameter gear.

 

[jim216]

Hi Rich,
There is a good article on converting the X3 to belt drive, he uses a 5mm HTD belt, the author recommends using 3mm drills I wound up using 1/8" drills and an 1/8" end mill, you will have to join the the Yahoo group to get the PDF or you can email me, this is John Stevenson's group, "Dick Stevens spindle drive conversion"

http://tech.groups.yahoo.com/group/X_Series_Mills/?yguid=391225240


Basically you drill on the PD then turn down the excess to the PD. ( I think I said the correctly). Or you can use an end mill and mill from the side on a blank that is already the correct diameter.

Caution if you decide to convert your X3 the authors center distance was different than mine I had to use larger gears for the same size belt, didn't want to wait for a smaller belt.

 

[firebird]

Hi

Interesting observations Admiral_dk some food for thought there.

Hi Jim, that's a clever way of getting the round bottom to the teeth. I'm not converting my mill though at the moment I'm just using the belts for another project. More food for thought though.

Thanks guys

Rich

 

[firebird]

Hi

A quick question

On a timing belt programme I'm looking at for a 24 tooth gear it gives these sizes

Pitch Circle Diameter= 9.549 mm

Outside Diameter= 8.999 mm

What does that mean??

Do I turn the blank to 9.549 or 8.999??

Cheers#

Rich

 

[Brian Rupnow]

According to the dimensions in reply #11, the pitch diameter is larger than the outside diameter. The FINISHED OUTSIDE diameter of the blank is going to be 8.999mm.

 

[tel]

????????????

24 teeth on an od of 8.999mm (0.3543")? That comes out at a tooth pitch of something around 1mm - never seen a belt that small.

 

[firebird]

Hi

sorry Tel

I forgot to put the belt pitch in

This is for 5mm pitch


Pitch Circle Diameter= 38.197 mm

Outside Diameter= 37.057 mm

Cheers

Rich

 

[HS93]

this may interest you, the tooth shape that is.

Peter

[ame]http://www.youtube.com/watch?v=ViN4t4YI_T4&feature=related[/ame]

 

[firebird]

Hi

Thanks Brian

Thanks Peter, that's really good. I was trying to work out how to make a form cutter just this afternoon.

I examined the belt a little closer today with some magnification. It's a 5mm pitch so I used a 2.5mm slitting saw assuming that the teeth and the hollows (hollows not the right word but you know what I mean) were the same size but the are not. The teeth are slightly larger so I re cut them with a 1/8 slitting saw (3.2mm)



The belt is now a much better fit. Far from right I know but at least the belt fits.



Cheers

Rich