Rhino Track Drive Pulley Improvements

This page contains information regarding the upgrade to the Rhino Pulley that took place during build season 2016. As of mid February 2016 all pulleys sent out with Rhino and related products are the improved "Rev4" version. For more information regarding our Rhino track drive, visit our product page: 


2016 Updates: 
Feb. 17th entry:

We sent an email to customers today stating these things:
  • A limited number of teams have reported cracks and breaks in the Rev 3 pulleys, approximately 3.7% of the total number of Rev 3 pulleys sold.
  • We conducted rigorous tests and were unable to consistently replicate the failures with the Rev 3 pulleys that have been reported to us.
  • Thousands of the Rev 3 pulleys are out there, many currently in use, with no signs of failure.
  • We stand behind our product and will replace any pulley that has evidence of cracks or breaks. Obviously, a pulley failure at a crucial time could be devastating. So, even if your pulleys have not failed, we will provide any Rhino Track Drive customer, upon request, up to 2 pulley sets per track drive (4 per robot).
  • For ease of reporting the failures we have developed a form for customers to report any pulley issues and request replacements.You can access the form here: www.andymark.com/RhinoPulley

Feb. 11th entry:

Late this afternoon, we received a box of the Rev4 version of the updated pulley halves. These new production parts will substitute the old version. We are shipping out spare pulley halves to the customers who have contacted us regarding cracked or fractured pulleys today. We are also sending out spare pulleys on Monday to customers who have emailed us and who have purchased pulleys or Rhino Track Drive modules which have not produced cracks or fractures yet. We are sending these spare pulleys out via FedEx Ground. (edit: This was a limited offer. We are now limiting the replacement pulleys to customers who have no experienced cracks or fractures, as stated in the 2-17 entry above. 2-17)

Rev4 of the pulley is now available at this web page: am-3298a.

Early next week, we will be tweaking our belt tension setup specification.

Feb. 10th entry #2:

Another update for today:

We've got good news and bad news.

The GOOD NEWS is that the pulley mold modification is done and we have some first run pulleys. They look good so far, but we are testing them now.

The BAD NEWS is that there was much sinking around the inner bore when we reduced that bore to 0.754". So, we won't have Improvement #5 as I mentioned earlier today. We will have Improvements #1-4.

Our mold tool has is a two cavity mold. The tool makers changed the inner bore to 0.754" on one of them, but they left the second one at the original size. They are changing the one back today and tomorrow so that they are both at the original bore size. This eliminates the change which helped users add in a bushing if they wished to do so. With these new Improvements (1-4), I don't think that this bushing addition is needed. It would have been nice to have it, but since it caused sinking trouble with the material, we quickly gave up on that feature.

Feb. 10th entry #1:

Danny and Jason, here at AndyMark, ran many tests on a chassis with new pulleys. The results from their test produced good news and bad news.

Good news: The pulleys they tested did not break.
Bad news: The pulleys did not break, therefore we don't know why our old pulleys broke and why some customers' pulleys are breaking.

Here is a record of the tests Danny and Jason executed:

"New pulleys and bearings were installed on the test mule after discovering the previous ones were broken. We tightened the track belt on one side as much as we were comfortable with and left the other side a bit looser. The pulleys were assembled with 3 washers under each screw to mimic the thread engagement of the new mold.

Here are the tests we performed in the order we performed them. In between tests we drove the track drive around on carpet and concrete. The test mule was weighted down to 120 lbs.

  • 30 Rockwall crossings, full speed on concrete. 
  • 30 Rockwall crossings with fully tightened front bolts, full speed on concrete.
  • Dropped from 2.5 feet onto concrete.
  • 10 Rockwall crossings, full speed on concrete.
  • Hammer to the side of front bolt 3 times each side.
  • Dropped from 2.5 feet onto concrete twice.
  • Ran into metal bar full speed five times.
  • 5 axial hits with weighted cart.
  • 5 front end drops from 2.5 feet. 
  • Crashing into various things in the warehouse. 

Throughout the testing we stopped between tests to look at the pulleys. The pulleys showed no signs of fracture externally on either side of the track drive.

Here is a video of our axial hit with the weighted cart."

We have received input from 9 customers stating they are experiencing cracked or fractured pulleys.

We are proceeding in making the mold improvements as shown below.

These Rev4 pulleys with the changes shown above will most likely be available by Monday, Feb. 15th.

If your team is experiencing cracks or fractures please send a note to sales@andymark.com, telling us which pulleys are breaking (1, 2, 3, or 4), your Rhino Track Drive AndyMark order number, and with pictures of the breaks. We will replace these pulleys with pulleys from the new mold.

We are hesitant to send out new pulleys to all customers due to the positive results we are seeing in #4 above. If customers want to contact us to get replacement pulleys, please do so by sending an email to sales@andymark.com. You will need to tell us your AndyMark order number (this saves us time) and tell us the number of pulleys requested.

Feb. 8th entry:

This is a page describing the improvements we are making to the Rhino Track Drive pulley. There has been some fracturing of the polycarbonate 20 tooth timing belt pulley out in the field. Each of these modules have 4 pulleys, so many applications with 2 modules use 8 pulleys. Between Feb 5-8, we received input from 4 different FIRST Robotics teams, pointing out that their pulleys are cracking and failing in the circular area around the bearing. We investigated our test mule Rhino Track Drive and found that some of our pulleys were also broken.

Customer Service:
If anyone has cracked hubs for these Rhino Track Drive modules, please send a note to sales@andymark.com. We will send out* replacement pulley halves. It will help us if you take pictures of the pulley fracture and tell us which pulley it is. Keep in mind that all of these pulleys are the same pulley, but we just want to know their position where they are used if there are cracks or fractures. Please refer to the pulleys numbered as such in this picture:

In house testing:
We have a test mule here at AndyMark that has been tested many times. Our initial tests (before kickoff) did not show cracking. We drove this mule over the wooden defenses after kickoff and it seemed to do fine. As it drove more last week, seemed to have more resistance to driving the tread. Today, when we drove it, the 120a breaker tripped. We took it apart and found that there were pulley fractures on both sides, on pulleys 1 and 4. Pulleys 2 and 3 did not have fractures. They had a few hairline cracks, but nothing was wrong with pulleys 2 and 3 to affect performance. Here is a picture of our pulleys we took off today (2/8/16):

This picture shows that the front (#1) and back (#4) pulleys were fractured. Since the middle two pulleys fared much better, we think this fracturing is due to the higher forces on the front and back pulleys with regard to belt tension. We are building up the test mule with new pulleys in order to make a test today. We will be testing the chassis with a slightly looser belt tension to see if we can find a belt tension sweet spot between the belt not falling and the pulleys breaking.

Pulley improvements:
The bearing fit is good, in our opinion. It's a light press fit with no taper. The radius at the bottom of the bearing hole is smaller than the radius on the bearing. Since we are able to recreate this failure here, I don't suspect that a customer was inadvertently using thread locking material (which is always a no-no with polycarbonate).

Material: This material is black polycarbonate with no filler. We have tried fiberglass fill years ago with omni wheels, and we learned that while strength is increased with fiberglass, impact resistance is significantly reduced. We could try to make this part out of some sort of Nylon or a co-polymer using Nylon, but that would require an entirely new mold and would take much time to try.

Mold tool improvements: We can take away metal from the mold to add plastic to the pulley. We will look at improving the mold in 3 ways:
  1. Adding a rib in the pocket between the mounting holes
  2. Raising the counter-bored level for the screw head so that the top of the head is slightly under the face of the pulley (we believe that we can do this without changing the screw length)
  3. Increase the radius on the ribs supporting the inner circle bigger
  4. Retain a portion of the radial wall around the counter bored area for the screw head

Corrective Actions:
1. There is enough evidence here to justify that we need to improve this mold. Once we get a new part designed, I'll post a pic of it on this thread with the additions highlighted. Since our molder is here in Kokomo and works with us very closely, I am confident that we can make a mold improvement and have parts available within 8-10 days. Thank heavens this part wasn't made overseas!

2. There is a short term fix being discussed on ChiefDelphi which suggests that users install a tube and bushing into the center of the pulley to spread out the load away from the outside ball bearings. This bushing may help, but it also will add some friction to the system.

* - While you may need cracked pulleys sent to you very soon, you may also wish to wait until we get this mold changed and have new, stronger pulleys. Please stay tuned for timing regarding when we can do this.

Andy Baker
AndyMark, Inc.

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