For a long time, there has being a debate on which type of screws is better, and most of these arguments bother on the superiority of one ball screw to the other. Though Ground ball screws seem to have been the choice for applications that require higher precisions, the Rolled ball screws seem to be more preferred for economical applications, especially for the general industrial and automation applications. It should be noted that advancement in manufacturing over the years has made rolled ball screws more desirable than in mere pepper grinders. Regardless of your preferred ball screws, there are some basic facts that will help you make an informed choice:
Manufacturing of Rolled and Ground ball screw
Rolled ball screws are manufactured through the pushing of each uncut blank stock through some rotating tool dies and through a single operation. There is a cold process deformation procedure in which the blank material is shaped by the rolling tool dies that will form both the internal and external thread of the rolling ball screw. The deformation of the internal and external threads of the rolling ball screws remains the same. The rolling procedure in the making of this ball screw is quite efficient because there will be no removal of any material when the blanks are cut. The process of making the rolled ball screws is widely adopted industrially because of the higher rates of production achieved.
On the other hand, Ground ball screws are manufactured through an abrasion procedure that happens in a horizontal arrangement. Under this arrangement, the blank materials are made to rotate on their long, horizontal axis, and in-between the centres of two machines, while hard abrasive cutters made to cut few threads a time will be used in carving the threads out from the blank materials. The same procedure is used in the cutting of the internal and external thread. It is a progressive abrasive cutting method that ensures that the blank material is cut to its final length before the grinding is performed. One can make use of different cutter styles and sizes to grind the internal and external threads of the ground ball screws even though the process of manufacturing is the same.
The strength and weaknesses of Rolled and Ground ball screws
Generally, rolled ball screws tend to be stronger than ground ball screws due to the higher material strengthening deployed in its manufacturing. Similarly, rolled threads are more cost-effective or economical than Ground ball screws. The reason for this is that Rolled screws can be quickly fabricated in automated rolling equipment.
The problem with rolled ball screws is that any error in rolled ball screws is more difficult to fix because of the one-time manufacturing process. Rolled screws have rougher surfaces than ground ball screws. Rougher surfaces mean rolled ball screws may reduce mechanical efficiency due to the high friction encountered when it is deployed.
It should also be noted that rolled ball screws are more vulnerable to distortions in dimensions because of the die tooling wear effects. This means there could be an excessive backlash that will occur as a result of the mismatch existing between the diameter of the bearing and the screw shaft threads. Positional accuracies are also decreased with rolled ball screws.
The main disadvantage of Ground ball screws is that thread grinding process with ground ball screws will take more time, and that explains why they are more expensive than Rolled ball screws.
The Rolling and grinding methods
One of the best possible ways to distinguish between the Rolled and Ground ball screws, is the surface finishing created by their rolling and grinding methods. It must be noted that rough surface finishing can become problematic in ball screws because higher friction is created and that means the load-transporting balls will suffer more wear.
The grinding ball screws tend to produce a smoother surface finishing, but rolled screws will require can be polished in order to eliminate the oxidative layer that is created during the rolling process, and to ensure that it creates a smoother surface too. When comparing the surface finishing of rolled and ground ball screws, one must compare the polished rolled with the ground ball screw. It is the quality of the polishing that will determine the quality of the surface finishing. You can learn more about this difference on this page.
The difference between the application of Rolled and Ground ball screws
There is a distinct separation in the application of these two types of ball screws. The rolled ball screws are mostly used in applications that are non-precise, for instance in the picking and placing of certain devices as well as in actuators. Ground ball screws, on the other hand, are mostly used as drive mechanisms in inside precision machines such as lathes, grinding machines, and profile grinders. It is quite uncommon to see these two types of ball screws to be found in the same application.
The difference in Geometric tolerance between Rolled and Ground ball screws
The same reference centres are used for thread grinding and journal grinding of the Ground ball screws, and for this reason, it is easier to minimize the radial run-out and also keep the screw threads. In the manufacturing of the rolling ball screws, the end journals are simply machined and then ground once the threads have been rolled, thus maintaining its concentricity, and the run-out will become more difficult.
When it comes to choosing between rolled and ground ball screws, it all comes down to what you need them for. If you want a faster approach to threading with little concern for surface smoothness, you should consider Rolled ball screws. If you want a slower and more precision threading, then you should consider ground ball screws. On the budget side, rolled ball screws are much more affordable and they can be more suitable for some home-based applications. Ground ball screws are more expensive and could be the better option for industrial application where higher precision of threading is desired.