View Full Version : belt tension

profqwerty - n/a
2-Sep-07, 08:44 PM
hi all,

just a quick one - is there a definitive method for checking belt tension?

it can deflect a little bit, and i can twist it about 40 degrees. 1100mm duct.





andycollins - n/a
2-Sep-07, 08:48 PM
With an HTD belt that will be fine.



tonybroad - n/a
3-Sep-07, 07:59 AM
a general rule of thumb is if you can twist it 90 easily it's too slack, if you can get a note by twanging it it's too tight

yours looks fine

nickyd - n/a
3-Sep-07, 10:13 AM

You should be able to twist it 90 degrees but no more. A lot of people run them too tight which increases belt and pulley wear.

I personally would say yours is just a little bit too tight........


Sean Atterbury - n/a
3-Sep-07, 11:47 AM
Nice lookin craft, post a full pic for us to see when it is done.

Its all shinny http://hovercraft.org.uk/images/icons/smiley_icons/icon_cool.gif

Mart366 - n/a
3-Sep-07, 11:14 PM
looks similar to my tension at the mo http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

thats what ive been doing all evening, ive about 20-30mm deflection on the belt, and can twist it to about 40 degrees too

will have to do a comparison http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

quick Q, how many teeth on your bottom pulley? and where did you get it

it looks very similar to my bottom pulley, but mines a taperlock, and fits the opposite way to yours is , (yours is what i,d like:))


profqwerty - n/a
3-Sep-07, 11:17 PM
It's got one washer thickness for shims at the moment. without the washer it's too slack and with 2 washers it's twangy. I might be able to find something a tiny bit thinner i guess.

I'll just run it a bit and if it starts getting too hot I can reduce the tension. Do they stretch at all which may loosen it?

just another question:

the duct is ever so slightly oval shaped - it needs the top pushing towards the left (looking head on) to make it a perfect circle. Any ideas how i can achieve this please???

I have tried moving it with the flow straighteners but they don't do a very good job.


profqwerty - n/a
3-Sep-07, 11:21 PM
sorry missed the last post - i can't quite remember how many on the bottom pulley (the tops one's 54) http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

Its K&M pulley - Ken mounted it onto the bearing block, so i believe he machined it with taper..?

how do you mean it fits the opposite way?

profqwerty - n/a
3-Sep-07, 11:23 PM
sorry - it's 28 teeth i believe!

hope this helps:)

nickyd - n/a
4-Sep-07, 08:10 AM
Hi Toby,

Best way to get the duct as round as possible is unfortunately at the time of mounting it to the craft..... Although you may be able to use the flow straightners.

What to do is get a piece of thick ply or chipboard. I use 18mm. Then cut a circle out of it the same radius as the inside radius of the duct. I used a router to get as round a circle as possible. With any luck it will be a snug push fit inside the duct. You can then tap it into place and square it up. This will force the duct into a round.

If attaching the duct to the craft at this point then it should remain pretty much round when you take the disc out. You could try and hold it with your straightners though.

The other advantage of this method is that you can drill a hole through the centre of the disc and this can be used to line your shaft up in the middle of the duct.

Another way of keeping it round is to go back to your piece of wood and cut a ring out of it. The inside diameter of the ring should be the same as the outside diameter of the duct. The ring should be about 50mm wide. You then can glass this ring around the outside of the duct. Put it where the blades sweep as it gives it more strength in case of blade faliure.

This will keep the duct round and make it so much stiffer. However it will look pretty ugly so it may be worth then putting a duct cover/cowl on to cover it all up.

Hope this makes sense. Let me know if i need to do a little better at explaining it!


tonybroad - n/a
4-Sep-07, 08:15 AM
to try and get your duct a little rounder try cutting a disc of mdf or plywood the diameter of the duct where the blades run and make it so it's a push fit into the duct, also drill a hole so it fits your fan shaft, relax the bolts on your fan frame and wedge the disc in the duct

once it's in, tighten your fan frame and then fibreglass a band of 4x1" thick foam around the duct so it makes a box section around the duct in line with the disc, this will stiffen the duct up and hopefully correct the ovalness, leave a good 48 hours before you remove the disc


hope that makes sense


profqwerty - n/a
4-Sep-07, 08:19 AM
ok sounds good!

hopefully tomorrow I'll have easy access to a cnc router so will do that.

nickyd - n/a
4-Sep-07, 08:19 AM
Great minds think alike, ha ha!!

OSPREY - n/a
7-Sep-08, 11:46 AM
For H section pulley (tzr engine) what should be the degrees for belt tension ?

nickyd - n/a
8-Sep-08, 02:15 PM
As above. A 90 degree twist but no more.