How Angular Errors And Misalignments Damage Precision

In general, it is always a problem when assembling several components that the addition of tolerances leads to overdeterminations in the bearings. Because the distances between the bearing points change, a geometrical overdetermination, caused by position errors in linear technology applications, often has a very negative effect.

With motion threads in general and with ball screws due to the high precision in particular, misalignment and angular errors have a major influence on the service life of the screw drive. Accordingly, position tolerances such as the squareness of the screw-on surfaces, the coaxiality of fixed and floating bearing units in addition to the threaded nut centering must be designed and manufactured with high precision.

roller bearing

How a bearing failure can be prevented

Almost all systems with screw drives have additional linear guides or guide bushes to absorb lateral forces. These guides contain traps for any kind of misalignment. This results in sustainable manufacturing problems, especially for assemblies of larger dimensions, because the size of the components means that machining of the bearing points is no longer possible or the individual components can only be assembled on site. This situation is often found in steel and plant engineering. If the tolerances cannot be met, the surface pressure in the thread increases significantly. Because the force curve no longer acts evenly on the threaded nut, the forces on the thread increase sharply at certain points,which accelerates wear in these zones and reduces the service life of the screw drive. The drive torques required and the power consumption of the drive motor increase significantly.

Problems in the automobile plant

In the present case, the problem at a plant in the VW plant in Wolfsburg became clear. Cast aluminum blocks are cut. The ball screw drive of the dimension KGT-4040 with a nominal Ø of 40 mm and a pitch of also 40 mm with a length of approx. 3.5 m fell far short of the predicted life expectancy. This was due to coaxial errors between the fixed and floating bearing points and the housing of the ball screw flange nut. The compensating elements that have been on the market to date can pivot in any direction in the form of spherical supports and thus compensate for angular errors, but they do not simultaneously fulfill the task of accommodating a possible axis offset from the nut adaptation to the spindle axis. Axis offsets in linear technology have fatal effects.Because the closer a centered spindle nut moves to a bearing point of the threaded spindle, the more the forces in the threaded nut and in the respective bearing point increase exponentially in the event of parallelism errors. The challenge was to develop a suitable adapter for threaded nuts that can compensate for angular errors and axis offsets to a certain extent. So a component that balances all four degrees of freedom of a plane on the threaded nut. The adapter should be as compact as possible and matched to the load ratings of the threaded nuts and also be combinable with existing standardized pedestals.the more the forces in the threaded nut and in the respective bearing point increase exponentially in the event of parallelism errors. The challenge was to develop a suitable adapter for threaded nuts that can compensate for angular errors and axis offsets to a certain extent. So a component that balances all four degrees of freedom of a plane on the threaded nut. The adapter should be as compact as possible and matched to the load ratings of the threaded nuts and also be combinable with existing standardized pedestals.the more the forces in the threaded nut and in the respective bearing point increase exponentially in the event of parallelism errors. The challenge was to develop a suitable adapter for threaded nuts that can compensate for angular errors and axis offsets to a certain extent. So a component that balances all four degrees of freedom of a plane on the threaded nut. The adapter should be as compact as possible and matched to the load ratings of the threaded nuts and also be combinable with existing standardized pedestals.The adapter should be as compact as possible and matched to the load ratings of the threaded nuts and also be combinable with existing standardized pedestals.The adapter should be as compact as possible and matched to the load ratings of the threaded nuts and also be combinable with existing standardized pedestals.

Angle error solved

With the KKA cross-gimbal adapter, Neff presents a solution: Angle errors of ± 5 ° and axis offsets in X and Y of 15% of the nominal spindle diameter can be accommodated by the new adapter system. With a 32 spindle, this means a permissible axis offset of ± 2.4 mm. Due to the integrated slide bearing in the version with ball screw nuts, the cross-cardanic adapter is very torsionally rigid. By adding the tolerances in the bearing points of the cross-gimbal adapter, the axial play in the standard increases to 0.05 mm.

The cross-cardan adapters are available in sizes 16 to 80. The connection dimensions are matched to the dimensions of the ball screw nuts of the KGF-D and KGF-N series of the same sizes. Trapezoidal flange nuts from the EFM series can also be used. At Neff it is also the case with the cross-cardan adapter that the series presented describes the standard. In addition, the cross-cardanic adapters can be supplied specifically for customer-specific applications.

One of the advantages of the cross-gimbal adapter is the increase in the service life of screw drives and screw jacks. The compensation adapter also saves energy through lower drive torques, and the effort for construction and assembly is reduced if the user can do without his own design approaches for position compensation. (br)

Hannover Messe 2017: Hall 4, Stand E34