Ball Screws from Automotion Components

When selecting a ball screw some of the main factors to consider are:

• Maximum required travel speed
• Maximum axial compression (buckling load)
• Method of support of the ball screws
• Type of unit required, fl anged, cylindrical etc.

In general it is best to support the ball screws with our ball screw support units (L1388 to L1406) with a fixed end (generally where the motor is mounted) and a floating (support) end. The support units are selected to suit the loads likely to be required, the size of the ball screw (especially its core diameter) and the type of mounting required. Details of the machining required for each end of the ball screw are shown in the bearing mounts technical section.

The data table for the ball screws show the diameter, the lead of the ball screw (i.e. how far the nut travels for one complete revolution of the screw) as well as the mass moment of inertia (also known as the rotational moment of inertia) - this is the extent to which an object resists rotational acceleration about its axis.

Maximum speeds and buckling load data are shown in the technical pages. When using a ball screw the ambient temperature should not exceed +80°C.

During assembly, the parallel alignment of the guides should be ensure. The details on the concentricity of the ball nuts to the ball screws are shown on the technical pages. For linear guideways for use with ball screws please see our part numbers L1016 etc.

Mounting the nut on the screw

Sometimes ball screws are delivered with a separate ball nut. When mounting the nut on to the screw take care as if done
incorrectly the ball bearings may come off the ball nut.

Ball nuts should be mounted only with the help of a plastic mounting sleeve (delivered with the nut). The start of the thread should be aligned so that the seal and the internal parts of the nut are not damaged.

1. Remove the rubber washer from one side of the sleeve. Push on the nut with the sleeve on the end of the screw. Press the sleeve against the start of the screw thread.
2. Screw the nut onto the thread using a slight axial pressure, then screw the nut on for it’s entire length.
3. Remove the mounting sleeve only when the nut is completely threaded on to the screw.
4. Lock the nut on to the screw (to prevent any unscrewing) using an O ring or similar - whilst installing the system.

If the balls do unfortunately escape...

1. Pick them up (the nut is only compatible with the original balls). The load capacity can still be achieved if one or two balls are missing.
2. Carefully clean all parts, use the sleeve as a mounting jig and replace the balls.
3. Start with the lowest circuit. Insert the balls into the nut circuit - the sleeve prevents the balls from falling out again.
4. Do not place the balls in the empty circuit located between the two deflectors.

If you have any technical queries please call 01483 26 67 74.

	n1	n2
Fixed-floating	2	10
Fixed-fixed	4	20
Fixed-free	0,25	1,3

A ball screw of size 40ø. This has a core diameter of 35.8mm.
Distance between the support parts is 600mm.
Using a fixed and floating bearing arrangement.
Reading of graph 1 (fi xed-fl oating) the maximum axial load is going to be 50,000 N (approximately 5 tons).

Example:

Average speed and average load:

In the event of variable operating conditions (speed and load), the working life calculations will rely on the average values F_m and n_m.

For the average speed nm, in the case of variable speed, apply:

nm = average speed (rpm)
q = time (%)

For the average load Fm, in the case of variable load, apply:

F_m = average load (kN)
q = movement or time at constant speed (%)

For the average load Fm, in the case of both variable load and speed, apply:

F_m = average load (kN)
q = time (%)
n_m = average speed (rpm)

Nominal working life:

Working life L, expressed in revolutions:

L = working life (in rpm)
F_m = average load (kN)
C_a = dynamic load capacity (kN)

Working life L, expressed in hours Lh:

L_h = working life (h)
L = working life (in revolutions)
n_m = average speed (r.p.m)
OT = operating time (%)

Output Torque (Nm)

Output torque Mt, for the transformation of rotational movement into linear movement:

Resistive torque Mr, for the transformation of linear movement into rotational movement:

M_t = output torque (Nm)
M_r = resistive torque (Nm)
F = actual load (kN)
P = pitch (mm)
η = yield (approx. 0.9)
η’ = yield (approx. 0.8)

Output Power (kW)

Output power P_a.

P_a = output power (kW)
M_ta = output torque (Nm)
n = speed of rotation (rpm)

(For preloaded double nuts, take no-load torque into account.)

Permissible axial load - Double bearings

Supported each end by double bearings

Permissible axial load - Single bearings

Supported each end by single bearings.

Permissible axial load - Double/single bearings

Supported each end. Double bearings one end, single bearing the other end.

Permissible axial load - Double bearing one end

Supported one end only by double bearing

Critical speeds of ball screws

As with any rotating shaft a ball screw has a critical speed - which is it’s harmonic vibration.

Critical speed is a function of the diameter and the length of the ball screw, as well as the mounting (support) configuration. The axial clearance of the nut has no influence on the critical speed. We recommend to limit the speed to less than 80% of the critical speed.

The critical speeds are shown in the graphs below.

Critical RPM - Double bearings

Supported each end by double bearings

Critical RPM - Single bearings

Supported each end by single bearings

Critical RPM - Double/single bearings

Supported each end. Double bearings one end, single bearing the other end

Critical RPM - Double bearing one end

Supported one end only by double bearing

Axial runout

Flange Ø	Max. axial runout
16-32	20
32-63	25
63-125	32
125-250	40
250-500	50

Concentricity

Flange Ø	Max. concentricity error
16-32	20
32-63	25
63-125	32
125-250	40
250-500	50

Parallelism (for preloaded units)

Motor torque and output power