In the following article, you will learn everything you need to know about mounting screw drives. In addition to a general overview, you will find key information on the subject of assembly tolerances. You will find assembly instructions in the final part of this article.
General information
Before and during the installation of ball screws, you should be particularly aware of one thing: Ball screws are only suitable for the transmission of axial forces. Radial forces and moments, on the other hand, must not act on them under any circumstances. They represent undefined loads, for example due to tilted installation, and lead to premature failure of the product.
Assembly instructions
The assembly of ball screw drives is generally straightforward. The main assembly steps are listed below:
Assembly instructions for the nut 1) Remove the rubber ring on one side of the mounting surface or the cable tie. 2) Slide the nut with the mounting sleeve over the shaft end. 3) Press the sleeve against the start of the thread. 4) Turn the nut with slight axial pressure on the thread. 5) The entire length of the nut must be turned onto the spindle. 6) Do not remove the mounting sleeve until the nut is fully seated on the spindle thread. 7) Secure the nut against running off the spindle (with a rubber ring or axial locking of the sleeve). You can find more information on assembly here in chapter 3. |
Although the assembly is relatively simple to carry out, typical sources of error should be pointed out. As separate ball screw nuts are supplied on mounting sleeves, the main error is to remove the mounting sleeve too early or to remove it incorrectly when the nut is hooked. The mounting sleeve must not be removed until the nut has been completely screwed onto the spindle. Otherwise, balls may be lost or enter the “dead gear”. A “dead end” is a section of the thread profile of the nut that lies between two deflections and is not filled with balls. If balls get into the “dead end”, they cannot circulate, which ultimately leads to the nut jamming.
Suitable lifting equipment must be used when transporting and handling long ball screws. There should be several support points along the screw length to prevent the screw from sagging or deforming. In addition, all mounting surfaces of the connecting structure must be burr-free and clean. To prevent contact corrosion caused by hand perspiration, dry and clean gloves must always be worn during the assembly process.
Assembly tolerances
Three directions must be taken into account for the assembly tolerances. These include the perpendicularity of the connecting structure to the spindle axis, the height offset ∆H of the bearings in relation to each other and the lateral offset ∆A of the bearings in relation to each other.
On the one hand, the tolerance depends on the preload, because the more preload there is, the fewer tolerances are available. With increased preload, the frictional torque increases, and a higher drive power is required. Another factor is the length of the spindle. A diagram can then be used to determine how many µm of assembly tolerance must be maintained for each individual case, depending on the preload and spindle length.
This diagram shows the assembly tolerance taking into account the preload and spindle length.
Last but not least, the following instructions must be observed when installing ball screws.
The ball screw must be aligned parallel to the existing guides. The screw is precisely aligned if the torque is within the permissible limit values over the entire stroke after installation. In the event of any deviations, optimization is possible by subsequent alignment in the end positions. In this case, no attempt should be made to move the nut with increased force. Furthermore, it must be ensured that the permissible tightening torques are precisely adhered to and not exceeded for all screw connections. It should also be noted that nuts that are not initially greased must be sufficiently lubricated before the unit is moved for the first time.
In addition to general information on screw drives, you can also find out everything you need to know about their calculation principles at linearwizard.com.
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