Acetal VS Bronze Nuts


At SourceRabbit we love the Leadscrew and nuts method for transferring rotational motion into linear motion. It is a very simple and effective method thus makes it a strong candidate for use in linear travel CNC applications.

So far in our machines we use steel Trapezoidal Leadscrews with 2mm pitch. On our tests we find these leadscrews very good and accurate and we will continue using them in our products in the future but... because there is always a room for improvement we decided to improve the quality of our linear motion systems by building our own leadscrew nuts!

After a lot of tests with the most common materials for trapezoidal leadscrew nuts, Bronze and Polyacetal, we decided to make our new nuts from Polyacetal. Polyacetal is an engineering thermoplastic used in precision parts requiring high stiffness, low friction and excellent dimensional stability [1].

There are several advantages and disadvantages to using this trapezoidal shape leadscrews when it comes to travel functionality. Because trapezoidal leadscrews have a 29° pitch angle, there is a larger surface area along the lead that is in contact with the nut. This is in contrast with the square leadscrew, which has no pitch angle.

There are two main effects, the first of which is that the trapezoidal can bear larger loads, as the load is distributed over a larger area than in the case of a square lead.

The second is that there is significantly more friction present in a trapezoidal lead system than there is in a square lead system. This causes the trapezoidal leadscrew to perform at a lower efficiency than the square due to frictional losses, which then become heat.

As a result, it is tricky to use trapezoidal in applications that involve continuous power, as this could cause overheating. However, this friction makes the trapezoidal leadscrew preferable for applications where the load must be held stationary or be suspended, as the additional friction prevents slippage.


Bronze & Polyacetal Nuts Comparison

Due to these particular characteristics of the trapezoidal lead shape, when choosing between polyacetal nuts and bronze nuts, the application must be kept in mind.

Bronze nuts have an advantage in larger loading situations due to the physical properties of bronze. Bronze is a much stronger material than the resins commonly used for polyacetal lead nuts. As a result of the difference in strength between the two types of materials, a bronze trapezoidal nut has roughly twice the operational load rating of a polyacetal nut of the same dimensions, and four times the static load rating.

This means that while a bronze nut can be used to move twice the load that an identical polyacetal resin nut can, it is able to suspend four times the stationary load that the same polyacetal nut can.

For any application wherein the load will be significant, the bronze nut is the frontrunner. However, bronze nuts are at a disadvantage to polyacetal when it comes to lubrication and power transfer. All bronze nuts require lubrication, whereas the polyacetal nuts do not. Due to the low coefficient of friction of resins, polyacetal nuts do not require lubrication and have a longer lifespan than bronze nuts. Further, they operate at a higher efficiency than do bronze nuts, and are less susceptible to overheating. As such, nuts manufactured from polyacetal resin are ideal for applications involving light loads and high travel. Polyacetal nuts are also more commonly used in the manufacture of anti-backlash nuts, since they lend themselves better to high travel applications than do bronze nuts. A nut without backlash is key to the precision of the instrument or machine using the leadscrew.

In the case of a CNC linear travel system, if the standard loading is within the loading parameters of an appropriately sized polyacetal nut, the polyacetal nut is preferable because of the travel and continuous power delivery. If it is necessary to use a bronze nut, it should be well lubricated. In either case, an anti-backlash nut is crucial. They are manufactured in standard sizes, and can also be made from two nuts and a spring without much difficulty.


Conclusion

The fact that polyacetal nuts can work continiusly without producing a lot of heat and the fact that they dont need often lubrication lead us to select the Polyacetal as the material to produce our new anti-backlash nuts

Load Handling Needs Lubrication Lifespan
Bronze Nuts Can handle large load Often lubrication Short
Polyacetal Nuts Small to medium load Minimum to None Long

References:

https://en.wikipedia.org/wiki/Polyoxymethylene
Acetal VS Bronze Nuts Acetal VS Bronze Nuts Reviewed by SourceRabbit on January 12, 2016 Rating: 5

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