I have a hypothesis I wanted to share with the community. I’m hoping this doesn’t cause too much of a stir. I intended to wait until I had a more detailed write up with diagrams put together to better illustrate my point before I started a discussion. However, I sense a failure cluster is beginning to form and I decided that getting this out in the community is more important to try to prevent further accidents/lost work.
I believe the recent problems we are experiencing with drive motor gear failures are related to the load the drive motors are experiencing at certain locations on the cutting surface. I also believe that the default frame design is the root cause of the problem. It is also a problem with custom frame designs where they exhibit similar characteristics to the default Maslow frame in one regard; angle of the drive chains as measured from the horizontal. I further believe that operating time at certaing locations of the cutting surface is a variable that needs to be considered.
As a brief primer for those not familiar, the sled weight hanging from the drive chains is a classic example of a load suspended from two fixed points. If the load is centraly located between the points and the two suspension devices (chains) are 90 degrees from the horizontal then the load is equaly divided between the chains. For example, if the load is 14kg then each chain will support 1/2 the load or 7kg. If we were to increase the distance between the anchoring points, we would decrease the angle of the chain to the horizontal. As a consequence, we have to increase the amount of x force in order to maintain the y force. This results in an increase in load on the hypotenuse (chains). Remember, at vertical the load on each chain is 1/2 the total load. As the angle of the chain decreases, the load placed on each chain increases from a factor of 1/2 to a factor of 1. This point is known as the critical angle and occurs when chain angle reaches 30 degrees from horizontal. At this point, each chain is experiencing a load equal to the total load of the sled. For example, if the sled weighs 14kg then each chain will see a load of 14kg.
Here is a crude example of what I am trying to say. It’s intended to describe a load suspened from a single point to two points on the load but it works for loads suspended from two points two a sigle point on the load if we flip it upside down.
As the angle decreases below 30 degrees, the load on the chains increases at an alarming rate. In our example, at 30 degrees the load on each chain is 14kg for a 14kg sled. At 27.8 degrees, the load on the each chain is aproximately 30kg. This is also the rated load of the drive motors. So, the load on the chains almost doubled for a decrease of only 2.2 degrees.
Again, here is a crude example. When fliped, it works to illustrate my point. The important takeaway from the sketch is the loads expressed for various angles as some fraction of or multiple of the total load:
As an exercise, I did some rough calculations using the design parameters of a users maslow. Keep in mind that my calculations do not include the amount of reduced load that is carried by the frame due to the incline of the frame. They also do not factor in the amount of increased load due to the friction of the sled on the cutting surface. They also do not factor in any counter weight that may be used. So take all I am about to say with a grain of salt.
Based on the reported numbers of the user, when his sled is at the top center of the cutting surface the chains had an angle from the horizontal of aproximately 16 degrees. I calculated that the load placed on EACH CHAIN at this location is aprox. 50.1kg.
It’s much worse when the sled is at the top left or right corners of the cutting surface. In these areas, the far side chain has an even shallower angle than when the sled is in the top center. Here, the chain angle is only 9.3 degrees. I had to calculate the 9.3 degrees based on the information the user provided. The resulting load placed on this chain, in this position, is aproximately 88kg PER CHAIN. I suspect (and it’s still a hypothesis) that this user is exceeding the load rating of the motors by a factor of 2.93 when the sled is in the top left and right corners of the cutting surface. This is probably why this users gear box failed when and where it did. The motors are experiencing the highest loads when the sled is in the top corners. As an asside, I am curious to know where the sleds were at the time of failure for the users who are experiencing them. My guess is that the sleds were at the upper half of the cutting surface and more likely in the upper corners. It’s not scientific. Once worn the gears could fail anywhere, but most likely will when they are under the highest loads.
I suspect the default frame design is the root cause of the problem. The user I spoke with and the numbers I used in my examples above came from his machine which he has reproted to be the default maslow frame. If so, the design of the frame is allowing the chains to regularly exceed the 27.8 degree limitation. Of course this is true for ANY user designed frame that allows for the exceeding of these limits and therefore may not be limited to the default Maslow frame. Time of operation in the areas where this limitation is exceeded is a secondary consideration. More time spent at these locations, the more wear on the gears. This also explains why ALL of the teeth are severly worn. Some may eventualy fail first but every picture I have seen shows teeth that are worn to knife edges and lubricant that is full of fine particle debris. This is indicative of gears under prolonged, high loads, not suffering singular, high impact loads.
Unfortunately, It is not practical to design a frame that prevents operation with a chain angle that does not exceed the 30 degree limit. I tried a couple of month ago. The physical space and chain lengths requied in order to cover the 4x8 cutting surface is not practical for most of us. Therefore, here are some potential fixes and workarounds:
- Avoid areas where the 30/27.8 degree limits are exceeded.
- Limit the time of operation in the areas where the limits are exceeded as much as possible.
- Shrink the size of the cutting surface to allow for a reasonable physical footprint to acomplish #1.
- Employ a counterweight system to better balance the loads.
- Source replacement gears made of a harder material that can withstand the forces experienced.
- Source motors/gearboxes with more powerful and robust units.
Alternatively, we can just accept that these are cheap motors and cheap gears and we get what we pay for. Consider the gears to be a consumable like toilet paper. Have a ready stock of replacement gears available and be prepared to replace them often and prophylactically to avoid a failure induced work piece spoilage.
Again, all of the above is based on some rough calculations that did not take into account several other factors. I am working on a white paper to illustrate my point better and to include the missing variables. Given the rash of failures being experienced by the community, I felt that it was more important to have this discussion sooner rather than later.