When machining a bearing at home often “DIY” fabricators will not consider that plastics have different tolerances than metals. Even more importantly, consideration if an application requires a press fit or running clearance isn’t considered at all. But it really should be to have a properly functioning bearing! To provide complete assistance on this topic we need to first define “press fit” and “running clearance”. Press fit is most often on the outer diameter of the bearing and it is a small amount of extra material on top of the designed diameter of the bearing to allow it to be forced into its mating partner (perhaps a wheel) and “stick” there without rotating. Frankly, if that is the type of application your bearing is going into – you need to be concerned with a press fit.

Same goes for a running clearance. In most cases the shaft in the center of your bearing needs to spin freely right? Did you account of that in the design of your bearing or did you plan to push through a 2″ inner diameter bearing over a 2″ shaft? What you’re most likely going to have is a shaft with a quasi-press fit that sticks on the plastic. Instead you need to factor into your design some extra room on the inner diameter (ID) of the bushing.

At this point you’re probably wondering how much press fit or running clearance? Redwood Plastics offers a handy online machinist chart showing guidelines for just that. You can find the chart here: https://www.redwoodplastics.com/brochures/Machinist-Chart.pdf

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Admittedly plastic welding is a fabrication technique that is going to be attempted by few “Do-it-yourselfers”. Plastic welding will require the purchase of special tools and filaments which are not always easy to come by.  In addition, since most plastic welding is done by specialists, it is difficult to get fabrication tips from plastics distributors or manufacturers. You’re likely going to have to search the internet for ideas and while we cannot vouch for the tips, YouTube seems to have several good videos to get you started off right.

In particular the company “Techspan” which manufacturers plastic welding equipment has, what looks to us, as a good 4-1/2 minute video on the basics of plastic welding. In particular we like that it talks about preparing the plastic for fabrication/welding (something often overlooked in these types of videos) and it goes into topics such as “tacking” which is welding two pieces on a 45 degree angle to each other. We’d prefer to let the video speak for itself and we’ve linked it below:

 

Quadrant Engineering Plastics is a major manufacturer of industrial plastics. While they do not sell directly to end-users, and instead sell through distribution, they still invest in many resources for end-users to help them reach their goal with their applications. One of the best is the “Machinists Toolkit” which is available by clicking here. This gives a variety of tips from what coolant to use, to tool tips and even troubleshooting specific issues that come up. It also gives feed rates and end milling/slotting guides for various plastics.

One handy section the toolkit has is a rating of the machinability of various plastics on a rating system. Acetal is usually the best where tight, critical tolerances are involved. But like with all plastics, there are situations where acetal is not ideal. In those cases when reviewing the various options you need to know what the “next best option” is. More useful still, is on the left side of the toolkit page is a link to the chemical resistance chart. This will allow you to look up alphabetically various plastics and their resistance to various chemicals. Please do not think that just because plastics in general are resistant to many chemicals, that a given plastic will be resistant to chemical exposure in your application! And do not assume that just because a chemical is “household” that it will not attack your plastic – it might, so do not assume!

Finally, Quadrant has also released a short video giving some machining tips. While the video says it is geared for a few high performance materials, much of the advice given is relevant to any home machinist working with plastics. That video can be seen here:

 

Something a little different today. We found a short (3-1/2 minutes) video tutorial on how to make a corn starch based bioplastic. None of the ingredients are toxic and most are what you would have around the house. It’s easy to make, simply requiring a pot and heating element. It’s a good introduction into the world of bioplastics which is a rapidly growing segment of the plastic market. Large companies such as soda pop manufacturers and other food processors that currently use a lot of plastics in their packaging are looking for biodegradable and environmentally-friendly plastic options.

This particular project would seem best for children interested in science. The goopy starch-based plastic can be used in simple molds or laid across stencils but it doesn’t seem practical for too many functional products. Its resistance to wear and properties seem to be similar to LDPE once cooled. To be honest the examples that the host of the video shows at the end are…Underwhelming. But since the basic mixture for this plastic is so easy to do it would hopefully inspire in your mind some better things to do with it.

We’ve posted the video here:

FRP wall panels are a great product for the DIY community. The panels are strong, long-lasting, grime and vandalism resistant. Excellent for applications such as bathrooms, garages, “mud rooms”, or areas animals are kept. FRP wall panels have a relatively economical cost compared to industrial FRP panels, are widely available, and easy to install. As with any project, preparation is key to saving money and prevent disappointment with the final result.

To start the first thing you want to do is draw a quick sketch of the area to be covered. Mark each wall with how many feet it is in length. FRP wall panels come in 4′ x 8′ and 4′ x 10′ panels and the next thing you need to do is figure out how you will orientate the panels. Start at the floor – will they be arranged to provide more height by being set side-to-side by their width? or by their length to provide 4′ high coverage? In many applications, the panels can be placed on their side to cover more area and reduce costs. You should now be able to figure out how many panels you need.

Next, figure out how much FRP adhesive you need. The adhesive comes in 4 gallon buckets and covers 200 square feet. simply add up the total area of the panels you need and divide by 200. For example, if your project requires 12 4′ x 10′ panels that is 480 square feet / 200 = 2.4 or 3 buckets of adhesive will be needed. It’s always good to have some extra so never round down!

Following this step you need to figure out how many inside corners/outside corners/j-trims you need. Inside corners and outside corners are PVC plastic dividers for FRP wall panel that are purchased alongside the panels. This is pretty simple as you just count how many corners you have. The trims and corners all come in 10′ lengths. The j-trims require a bit more thought. These are also PVC but are used to join two panels where there are no corners. You need to be able to sketch or visualize where your panels will buttress up against each other along the stretches of wall and be able to count up what you need.

And that’s pretty much it! At this point you should have a clear idea of the scope of your project and all the requirements for your FRP panels and how many accessories you need. The next step is to simply contact your plastics distributor and request a quotation. Please give clear and specific requests including relevant quantities for all required components. That will ensure a quick and accurate quotation.

 

Sheave Design: Advanced

Posted: August 9, 2017 in Education
Tags: , ,

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.