Ball Screws for Heavy-Load Applications

Design

In many cases, special design measures must be taken so that the load can be safely borne by the ball screw. The load direction in all load phases is very important for this purpose. Unlike an alternating stress where the axial load force makes its way from both sides into the ball screw, with a unilateral load of a double nut the preload nut may be done without so that all loaded turns in both halves of the nut are arranged to bear the load and can be used. It is also advisable for a quick and easy assembly to provide a smaller preload nut that keeps the nut system centered and together. In this way, a normal double nut configuration results in a triple nut with extreme load bearing capacity that is mostly used in vertically arranged ball screws, since there is almost always a unilateral load direction here.

Due to the high load (and the associated danger of blocking the balls), the ball chains and the inner friction in the ball screw must be kept as low as possible. This is why heavy-load ball screws are always designed to be friction-optimized.

Friction Optimization Through Deflections

The friction-optimized design of the ball screws can either be achieved through individual turn deflections or through total deflections with separating balls. The use of separating balls benefits the running behavior of the ball screw, but also leads to a loss of load bearing capacity due to the reduced number of bearing balls.

Nut Housing Design

In addition to the power transmission through the balls, the dimensions of the nut housing must also be designed for high loads with heavy-load ball screws. That is why such nuts are usually designed with larger component walls (flange thicknesses) and more mounting screws. Depending on the nominal diameter, very large nut systems are thus created whose total length may be up to 800 mm.

Speed Characteristic Value

The speed characteristic value of d_n x n (nominal diameter x speed) < 90,000 applies as a reference value for heavy-load ball screws!

Special Designs

A.MANNESMANN supplies these precision ball screws for heavy-load applications in the nominal diameters of 63, 80, 100, 125 and 160 mm with all the common thread pitches and in the accuracy classes better than IT 5.

Of course as a manufacturer, we also produce special diameters. Please just send us your inquiry.

Characteristic Values of the Ball Screws

In addition to the dimensions, the load ratings, the nut rigidity and the idle torque are the classifying characteristic values of ball screws. The load ratings describe the load bearing capacities of the ball screw during movement (dynamic) and at rest (static). The dynamic load rating is also the basis for calculating the service life.

The nut rigidity is the measure for the axial force, which is required for deformation in the ball transition contact. The idle torque is the characteristic value for the internal system friction. Together with the nut rigidity, the preload can also be closed via the idle torque.

You will find the characteristic values in the technical data sheet of each ball screw.

Rotatory Mass Moment of Inertia

The rotatory mass moment of inertia (J_s) of the spindle is usually required for designing the motor. With driven solid spindles, this (with a good approximation neglecting the thread geometry) corresponds to the cylindrical spindle body with respect to its axis of rotation.

Here the ball contact diameter d_c on the spindle with the 4th power, the spindle length l_s and the density of the spindle material r_s are decisive. The mass moment of inertia J_s is usually specified in kgcm². If the rotatory mass moment of inertia J_s is related to the spindle length of one meter, then it can easily be converted by multiplying the ratio of the existing spindle length with one meter.

Spindle Torsion

The spindle is normally driven by a motor. The motor's drive torque is guided through the spindle to the nut and there converts into an axial force. In the slim spindle, this results in a torsion due to the torsional moment that exists. The torsion length, the torsion angle and therefore also the torsional rigidity depend on the axial nut position on the spindle. Since this can change variably in operation due to the axial displacement, the torsional rigidity is also not a constant characteristic value.

The maximum torsional stress of the spindle (can usually also be assumed to be a circular full cross-section) can be determined from the torsional moment or torque (T) and from the polar modulus of resistance of the ball screw spindle (W_ps) or the spindle diameter.

The torsion angle (torsional angle) is dependent on the spindle length or on the position (I) where the torque acts and on the shear modulus (G) of the spindle material. To calculate the torsional angle, the moment of area of the 2nd order (I_ps) and the torsional moment (T) are also required.

Since the effective position (I) varies depending on the axial travel movement of the nut, the torsional angle (j) is also not a constant value, but rather is a function of the length (I). However, torsional angle is also directly dependent on the time variation of the effective torque, whereby it is also to be seen as a function of time. That is why torsional vibrations that temporally vary depending on the torsion are created, which also act in the axial direction through the ball screw.

Tensile and Compressive Load

In addition to the torsional load, a driven spindle is mainly loaded by tension or pressure. In both cases, the tensile or compressive strength of the spindle are the load limit values.

When evaluating the load, it is important to take the correct "smallest" spindle cross section into account. Only the cross-section that lies in the spindle's distribution of forces is relevant for determining the tension. With the tensile load, the determining spindle cross section usually does not lie in the area of the ball screw, but rather in the area of the mounting thread of the bearings. The buckling stability is also to be noted, since there is a risk of buckling in the event of a compressive load on particularly long spindles.

Characteristic Value Calculations

We will use this to calculate the following characteristic values for you:

You will then receive the calculations and results by e-mail as soon as possible.