Sheave Design: Advanced

Posted: August 9, 2017 in Education
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On our previous post we taught you the bare minimum that is required to design a simple sheave. Now, we’re going to discuss some ways to add advanced features to your sheave. Firstly, is how to calculate webbing. “Webbing” in regards to a sheave is removing material outside of the hub and outer diameter to lower the overall weight of the sheave. A webbed sheave would look like this:

 

 

 

 

But how do you know how much material you can machine off? The math is actually quite simple: W = 1.2r where W is the “minimum web thickness” and “r” is the radius of the rope or cable. So for example, based off a 1″ diameter rope the minimum web thickness is 1.2″. This naturally segways us into a discussion about the hub. As you can see in the picture of the webbed sheave, the hub has to be wider than the webbing. But by how much? Again, the math is simple: H = 1.5b where H is the “hub width” and b is the bore size. Lets assume the bore is 1″ again, you would then require a hub no thinner than 1.5″. Typically the hub is at the very least as wide or wider than the rim, so always run off that rule of thumb.

Finally, the last calculation is to figure out a press fit if you’re going to push a bearing into your sheave bore. You need to know how to precisely bore but leave just enough room that the bearing won’t slide around. That is done as follows:

 

This will give you the bore diameter you require to fit your bearing. You now have all the tools to make not only a sheave but a fairly complex one if you’re so inclined. If you want to download the whole Redwood Plastics sheave design manual, where this information was taken, you may do so here.

If you need a quotation on some sheaves or sheave materials please contact Redwood Plastics.

 

 

 

 

 

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.

 

We have an unspoken rule we’re about to break here. This blog is about connecting the “do it yourself” (DIY) community with industrial plastics. Plastics that are not very well known outside of their specific industries, but offer a lot to the home handywoman or handyman. Industrial plastics are also known as “engineering” plastics and are a separate grade of materials than what makes up most consumer products. But we were trying to find a new application for summer and stumbled on something with “commodity” plastics, IE. plastic bottles, we wanted to share. Pretty sure this application would be difficult to do with the usual industrial grade sheet/rod/tube…And hey, it promotes plastics recycling! Always a good thing.

It’s a home-made “air cooler” using nothing but Sprite bottles, tubing, a drill, a few pieces of wire, a glue gun, and an oscillating fan. We haven’t tested it (like most applications we highlight) so don’t take our word for how good it works. It’s a short video we wanted to share as North America starts to experience the heat waves that hit with late spring and summer. One change we’re confidant in recommending; however, is the use of a 2 liter bottle as the drain bottle strapped to the fan (you’ll see in the video) as it makes little sense to have two one liter bottles both drain into another one liter bottle. Based on the volumes at play, that would result in a bit too many times having to get up and drain! The full video is below and is about four minutes long, hopefully it works for you!

Nylon is a fairly popular plastic used by the DIY community: and it should be! Versatile, strong, and available in small quantities it offers many properties valuable to the public’s plastic applications. In most cases, nylon will be an ideal material for mechanical parts such as sheaves or bushings. But it isn’t perfect and it like anything it has its drawbacks in certain applications. Here are some tips on using nylon that will serve you well.

1.) Don’t use it in the cold

Ok, well you can but you just need to be aware of impact. Nylon gets brittle in the cold, at about 5 degrees Fahrenheit and if it’s taking any sort of impact it could break. In fact inĀ  any application where you expect impact, use Redco Tuffkast instead (it has been developed to replace nylon in applications with impact.)

2.) Don’t use it in water

Unlike most plastics, nylon absorbs a significant amount of water – up to 4% of its volume in 24 hour saturation. For marine applications or those involving tight tolerances you probably want to go with an alternative material. Acetal is commonly specified in this case but Tuffkast may be an option as well, it depends on the application, so ask your distributor.

3.) Do use it under load

Nylon can handle 4000 PSI in application and this is one of the highest loads of readily available thermoplastics. This makes nylon excellent as a bearing, sheave, or other load bearing part.

4.) Pick the optimal grade

Nylon isn’t just “nylon”. It’s available with a variety of fillers: glass for dimensional stability, moly for high load low RPM applications, oil filled for reduced friction, UV stabilizers for outdoor use, and PVM (pressure velocity maximum) for the highest load applications. These fillers add only a minor cost, or in some cases cost the same as the natural grade, and are well worth it for certain applications. Unsure if you need a filler? Discuss your application with your distributor.

Even though three of these four tips are “don’ts” knowing when to avoid a material is just as important as knowing when to use it! Don’t get us wrong – nylon is a versatile, excellent, affordable plastic for a wide range of applications. Just keep in mind that it has advantages and drawbacks: just like every other material out there.

One of the few PTFE applications that brings in the DIY community consistantly to a plastics distributor is the application using sheet material cut into discs as mountings for telescopes. PTFE is the third slickest solid known to man and works well as a sliding surface. It does have very low load-bearing capabilities and is subject to “creep” (dimensional changes at rest) and also cannot handle much wear at all. In short, there are only specific instances the material makes sense – especially due to its high cost compared to most other plastics used in DIY applications.

We recently came across a blog posting from an amateur astronomer (“Chris B.”) chronicling his progress in making telescope mountings from PTFE. He gives some pretty handy tips such as how to cut the material precisely into the desired disc and to keep it from moving around. We don’t really have experience in this area so it’s probably best to let Chris’ work speak for itself. You can find the blog here: https://fullerscopes.blogspot.ca/2016/08/2-shaft-mouting-pt28-ptfe-clutch.html

We found a great little video on YouTube by user “Make:” which in just three minutes (before credits) gives all sorts of great tips for the DIYer using acrylic in their applications. It starts off with a good point that not everyone knows: acrylic comes in both cast and extruded forms. Yes, they have differences that are important in an application. Yes, there is a difference in cost. Other important tips include how to thermoform, glue, and most importantly – drill, the plastic. Drilling is especially important as the plastic can crack easily, so using lubrication and a soft touch is essential to prevent a wasted part. It will help you answer some questions you didn’t even know to ask prior to diving in: do you have all the materials you need? How are you planning to smooth the edges of the acrylic?

We’ll let the video do the rest of the talking:

Are you a “DIYer” who makes plastic bearings at home or perhaps looking into doing some? Not sure how to calculate a press fit or a running clearance for your bearings? Fortunately there is a useful tool available from Redwood Plastics availableĀ  on their website: the machinist chart for plastic bearings found here.

This chart provides valuable information for bearing manufacturing using Redco 750 or Redco nylon bearing materials. While many in the DIY crowd like to use UHMW polyethylene for everything, including bearings, this is not a good bearing material and has large and variable tolerances. The bearing chart is not intended to be used for UHMW bearings. Acetal is similar to nylon and therefore nylon’s values can be substituted directly.