One of the best ways to overcome limitations from critical speed is to rotate the nut and affix the screw to the load. The screw will translate, and the nut will be immune to the whirling and vibration problems associated with long, slender rotating shafts. In this design the nut is integrated with a sprocket and driven by a timing belt.
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Ball screws are ranked and rated according to factors like speed, noise, accuracy, and life expectancy. To qualify as “high performance,” they must achieve certain minimum levels across all such variables. As with most engineered systems, there is some give and take between various properties, but these can be adjusted to favor the application. Insight into some of the dynamics at work can help you figure out the best design.

In the fast lane

Faster machines save time, and time can be essential. “Fast,” for a ball screw, is anywhere between 100 and 200 m/min, the higher end of these speeds occurring under test-lab conditions. Of course, what we now consider quick will probably become commonplace in a year or two.

The angular velocity required to produce a given traverse speed depends on the screw lead. While greater leads lower the necessary rotation speed, they also require more driving torque, often necessitating a larger, more expensive motor. Furthermore, increasing the lead compromises accuracy. In practice, leads are best kept within 62.5 to 100% of the screw shaft diameter.

Ball screws are efficient at converting from rotary to translational motion (efficiency is upwards of 90%), but they also require various inventive means to transfer (circulate) the ball bearings from one end of the ball nut to the other. The impact of ball circulation on speed can be emphasized by a quick comparison with rotary roller bearings. The speed capability in bearings and ball screws is commonly given by the mean shaft diameter multiplied by the rotating speed (d_mn). Where a 40-mm shaft running at 5,000 rpm might not be a big deal with precision bearings, the 200,000 d_mn product is significant for a ball screw, presenting a considerable challenge to the recirculation system.

Shaft whirling and vibration at critical frequencies is a very imposing factor when it comes to ball screw speed. In a conventional arrangement, the screw spins while the nut translates. Long, slender screws are most susceptible to whirl; they have lower lateral natural frequencies (correlating to lower critical speeds) and tend to behave in a flexible manner.

To minimize constraints of critical speed, some ball screws employ hollow screw shafts. Pound-for-pound, hollow shafts are more rigid and have a higher natural frequency than solid shafts. However, the higher shaft inertia can impair performance in other areas of the system and may require larger drive motors. As travel length increases, the large shaft diameters needed to limit whirling can put a real strain on the rest of the system, including the nut, and there have been extreme cases where inertial effects led to failure after a few hundred hours.

To get around shaft whirl in some highspeed applications, the nut may be rotated instead of the shaft. While running a motor to the shaft is pretty straightforward, driving nuts can require a bit more arrangement. For example, in one method the motor is offset from the ball screw axis and turns the nut through a timing belt.

Put it right there

Accuracy and repeatability of a linear motion system relies not only on the ball screw but on the larger assembly to which it mounts. As for the ball screw, accuracy is instilled by closely controlling the lead and the ball circle diameter during manufacture. Normally, the minimum specification would be ANSI class 5, ISO class 3, or JIS class C5. Under these specifications, the lead accuracy would be between 12 and 18 μm per 300 mm of track.

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To ensure ball screw repeatability, zerobacklash or a light preload might be instated between the nut and screw. There are various methods to generate a preload or control backlash. Preloads are often stated in terms of the ball screw’s dynamic capacity and for high-speed units they are normally set in the range of 0.5 to 5%.

Among some high-performance screws, a four-point contact principle is popular. This relies on two contact points between the ball and screw thread, and two contact points between the ball and nut thread. With this technique the degree of preload can be varied by changing the ball bearings’ diameter by a few μm.

Another typical preload mechanism uses a split-nut design. The nuts are axially loaded, either pulled apart or together, creating ball contact that’s “mirrored” across the split. Holding the components to their outer or inner extremes of contact reduces the possibility of nuts or screws displacing axially when there’s no rotation.

Another way to improve the stiffness of the system is to apply pretension to the shaft itself. Not only does this take some of the slack out of the assembly, it also helps accommodate thermal expansion. Shaft expansion varies with the preload and the overall work being done by the ball screw; these factors affect heat generation. With hollow-shaft screws, external cooling is sometimes employed. For non-cooled ball screws, the amount of shaft strain applied from pretension is usually between 12 and 25 μm/m.

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