How to choose a proper servo coupling?
It’s a complex process to select a coupling for servo system, and a detailed understanding of the performance and application details of couplings will help to select suitable couplings. This paper tries to introduce the performance of different couplings to end users of servo coupling, points out the factors to be considered in the design and how to select suitable couplings for different application situations.
Helical slit flexible coupling
Helical slit flexible couplings are usually called as integrated slit cutting coupling or flexible coupling, they often have good performance and cost economy, and are regarded as preferred products in many applications.
The integrated design of a complete aluminum alloy bar makes it to have advantages of zero backlash drive moment and no maintenance. The helical slit flexible couplings can be broadly divided into two basic series: single head helix slit and multi head helix slit. It has large flexibility but low additional bearing load. Such a slit design is particularly suitable for handling the angular deviation and axial deviation, but not suitable for parallel displacement deviation, because the single head helix slit would produce multiple directional bending when handling parallel deviation, which may result in the excessive stress concentration and earlier damage of the parts.
Multi head helix slit coupling reduces the slit length without losing the ability to rectify deviation, the multi head helix slits are intertwined to enhance the torsional rigidity of the coupling, thus ensuring that the coupling can still withstand considerable torque when it has great rectifying capability. This performance makes it suitable for light load applications, such as the connection between the servo motor and the screw. However, such a design also has its shortcomings. The additional bearing load will increase as the size increases. But in most cases, as the installation error is smaller, the bending moment is also very small, thus ensuring low additional bearing load.
Cross slide coupling, the best parallel correction ability
Cross slide coupling consists of two hubs and a central slider. The central slider matches with the hubs through slight pressure, and such a combination ensures the coupling with a characteristic of zero backlash.
As time goes on, the slider may lose zero backlash due to wear, but slider is cheap and easy to replace, original performance can also be restored after replacement. In application, the sliding of central slider can adjust the relative parallel deviation of the shaft. The cross slide coupling has a high cost performance, which is a a great advantage of cross slide coupling. Three part design of the split restricts its axial deviation adjustment ability, for example, it’s not suitable for push-and-pull application. Meanwhile, as the central slider is floating, the movement of the two axes must ensure that the slider will not fall off.
Zero backlash plum blossom coupling, a coupling that absorbs impact the best
There are two types of couplings, one is the traditional straight claw type and another is the curve (concave) claw type zero backlash coupling. Traditional straight claw coupling is not suitable for the high precision servo drive.
The plum blossom coupling has good balance performance and is suitable for high speed applications (the maximum speed can be 40000 r/min) , but it can’t be used for large deviations, particularly the axial deviation. Larger parallel deviations and angular deviations will produce larger additional bearing loads than other servo couplings. Another concern is the failure of the plum blossom coupling. Once a elastic plum block is damaged or failed, the torque transmission will not be interrupted, the metal claw of the two hubs will mesh together to continue transmitting torque, which may cause problems in the system.
Disk coupling, high torsional stiffness, high speed
Differences between single disk coupling and double disk coupling is similar to that of single helix slit and multi helix slit, single disk coupling is not suitable for adjusting parallel deviations while the double disk coupling can bend in different directions at the same time, so it can bear parallel deviations.
The metal lamination is very thin and it’s easy to bend when the deviation load is produced, so it can withstand up to 5 degrees of deviation while generating lower bearing load at a same time. The metal lamination has a good torsional stiffness, just next to the bellows coupling. The disadvantage is that the disc coupling is quite delicate, and it’s easy to damage if it is misused or not installed correctly. So it is very necessary to ensure that the installation deviation is within the normal operation range of the coupling.
Bellows coupling, high torsional stiffness, high speed
The bellows coupling consists of two hubs and a thin metal tube, which are connected in a welded or bonded manner. The thin wall bellows make the coupling with characteristics of high sensibility and quick response, it’s an ideal choice for micro precision instruments. However, thinner tube wall also reduces the torque transfer capability, which makes it great limitations in practical application. Stainless steel bellows have greater rigidity and higher strength than nickel bellows, and it’s often made by means of hydroforming.
Solid coupling, high centering requirements for installation shaft
As the term suggests, solid coupling is a kind of coupling with rigid torsional rigidity, there’s no clearance gap even bearing a load. If there is any installation deviation in the system, the shaft, bearing or coupling will be damaged prematurely, that means the solid coupling can not be used in high speed environment, because high temperature may occur on the shaft when the machine runs at high speed, the high temperature causes the expansion and deformation of the shaft, but the solid coupling can not compensate for axial size deviation caused by shaft expansion. Of course, if the axial relative displacement deviation of the shaft can be successfully controlled, the solid coupling can also perform well in servo systems.