Belt Drive Idler Size

Does anyone know where I can get info on the minimum size Idler or tensioner pulley size to use with 3Vx banded belts. I see they go quite small with syncro and micro groove belts. Is there any info out there on Vbelts?

Attachment 4153

I have been having this same debate in my mind also. with on of the project that I'm in the middle of.
in the attachment below. it gives the minimum recommended pulley diameter of 2.20 for the OD. of the pulley, i am thinking that this would include the tensioner pulley as well.
this article also references 2 other articles by Bestorq.com "elongation method of tensioning" & "normal v-belt tensioning"
https://bestorq.com/InstallationMaintenance.asp

at the moment i haven't really found a recomended pulley size, persay. but I will be needing to build my own idler pulley later this spring. I would have to pull up my notes...
but i would not go any smaller that the OD of 2.20"
don't know if any of this information helps will help you out in your quest.
how much horse power & RPM's will you be turning?

Thanks for getting back Bart,
HP is 48 hp and 45 ft lbs at 5500 rpm. The drive is 2.66 to 1 it has an 8" pulley on reduction side and 3" pulley on drive. I am thinking about a 3.5" idler, I will increase the belt length so I can increase the wrap on the drive pulley a bit.

Ok I found this at http://bestorq.com/Library/TechInfo/techinfo/Idlers.pdf It recommends a minimum back side Idler of 4" for 3VX belts. I have ordered a 3.5" already, it will decrease belt life some but we will see. They do have a 100mm 4" as well but the 3.5 will fit better.
Attachment 4154
SVT MUSTANG COBRA, LIGHTNING, SHELBY 90mm DOUBLE BEARING 10 RIB IDLER PULLEY

glad, that you searched a little deeper, up until reading the link, I was going to turn down a 2-11/16 round stock, but it looks like I will be picking up some more round stock. Just a little bigger this time. It is always the little things, that can bring a project to a screeching halt.

Yeah I also ordered a 100 mm pulley of the same thing so I can see which will fit in place better, If I can I will try and use the 100 mm just because it will extend belt life a bit. It will be set up as an Idler and not a Tensioner, I have to add a bit of aluminum to the side of my Reduction drive plate to get it to mount, got a piece of 3/4" x 1.5" 6 inches long to add it on. The slots cut in the pulley should work to get at one of the bolts that hold the plate to the motor.

sounds like you are on top of it. it should be looking good when you are done.

I was looking at lightening force's web site and they have some good looking idlers there thanks for the info...I'd really like to retrofit something on my Circle S redrive with a tensioner instead of the hammering set-up it comes with..

Here is the Idler mounted on the drive in my work area.

Attachment 4198

looking nice, i was thinking of using a eccentric collar to to adjust the tension, I like how simple you kept it. does it give you enough slack to take the belt off when you remove the tensioner pulley?

I am still using the original means of belt tension. The idler is not adjustable it is fixed, to take off the belt it requires me to loosen the belt tension by 2 jack bolts that push up from the bottom hitting the backing plate. You cannot see that as I have only mocked up the engine inside my work shop on a table saw stand, the bottom plate is still on the boat. Once you remove the tension off the backing plate, I remove the idler pulley allowing the belt to come off. Below you can see how the original set up works. The drive pictured is a 1.69 to 1 I believe from Lonestar Hovercraft.
Attachment 4199

Nice job...That really added some area to the pulley's belt contact ...I'll be surprised if that hasn't solved the belt slippage problem associated with that style of redrive..

I ended up using the 4" pulley so I have a 90mm (3.5") pulley I don't need to keep. Anyone interested it is 1.75" wide and has the double 6203 2rs bearings. Will sell for $50.00 shipped TYD in the USA.
Will put it on ebay if no one is interested.

i don't disagree with you corky but i do think a shorter belt might have been less work. this setup looks just like mine from arrow props and the only time when mine slipped was break in period. i haven't adjusted mine in over 6 months and doesn't slip at all. it's the notched belt shown here but shorter and i love it. not like the whining circle s belt drive. i'm glad i gave it to my son.

The reason for the idler is I changed it to a 3" pulley, the normal sizes are 3.65" or 4.14" with a top pulley of 7", 1.69 to 1 or 1.91 to 1 ratio. The larger pulleys have more wrap on them, with less chance of slip. By adding idler I increased wrap.