Sheave Design: Basics

Posted: July 25, 2017 in Building & Design, Nylon, tuffkast
Tags: , , , , , ,

So you’re a “DIY’er” at heart. You want to make your own plastic sheaves but don’t know where to start. You’re not sure what material or grade is best for your application and you don’t know what is the minimum amount of “engineering” needed to make or procure a plastic sheave? This write-up will help you through that process.

First of all, material. Assuming your sheave is not going to take a lot of impact and is not used in a wet environment go with moly-filled nylon. This is the same nylon used on crane sheaves and is optimized for low-RPM, high load applications. If your sheave will take impact or be used in a wet environment we would recommend Redco Tuffkast. This is a co-polymer material which overcomes many deficiencies in nylon: Tuffkast can take impact and is better in wet or cold environments. It is more expensive than nylon, however.

After material selection you need to know these basics for the simplest design (a non-webbed, bearing-less sheave):

  • Bore diameter of the center hole.
  • diameter of the rope or cable to be used on the sheave.
  • Overall diameter of the sheave

Next you’ll have to do some very simple math. Firstly, to figure out how deep the groove in the sheave should be: (rope/cable diameter) x 1.75. This will give you the minimum groove depth you need, but in most cases just round to make it a little deeper and give yourself a safety margin. For example, if your sheave is 15″ in total diameter and you have a 1″ diameter cable. That is 1″ x 1.75 = for a required depth of 1.75″. But for the sake of safety margin you can make this an even 2″. The inner diameter of the sheave is now 11″. Please note that for the inner diameter you are taking that required groove depth x 2.

The last thing you need to consider is the thickness of the sheave. For most smaller sheaves just go with a 1/4″ wall thickness, these are the “shoulders” of the sheave on either side of the rope groove. So, for example, if your rope groove is 1″ wide, then you add another 1/2″ for the walls (wall thickness x 2) so you would have an overall thickness of 1.5″. The last thing to touch on here is the radius of the rope groove (the curve of the groove the rope sits in). this is almost always 30 degree and in rare cases, 45 degrees. Run with 30 degrees as a standard.

There are some guidelines for figuring out parameters for more advanced sheaves such as webbing or thickening the hub and we’ll discuss those next time in “Sheave Design: Advanced”.

For help with your sheave applications and to purchase sheave materials please contact Redwood Plastics.

 

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