Pierre C. Lemor
SKF Motion Technologies
Bethlehem, Pa.
Linear actuators come in many different designs, but for rugged applications planetary roller screws are often a best choice. They have the inherent capacity to withstand high accelerations, run at high speeds, and carry heavy loads for a long time.
These capabilities often bring comparisons with large ball screws and hydraulic actuators. Roller screws, however, stand up to adverse environments better than ball screws, and are not prone to leaks and unpredictable failures that can plague hydraulic cylinders.
Well-established techniques describe conditions for running roller screws for optimum efficiency, low noise, and long life. And now, for the first time, designers can predict reliability based on fatigue life.
Roller-Screw Basics
A roller screw includes three basic elements: a threaded screw shaft, a threaded or grooved nut, and a set of threaded or grooved rollers that engage both the shaft and nut. Rotating the nut causes axial shaft movement. Three different designs of roller screws are commonly available.
Planetary roller screws, produced since the early 1950s, have a threaded nut and rollers in addition to the threaded shaft, which permits planetary roller motion. This unique feature eliminates the need to recirculate the rolling elements, meaning fewer components and longer life.
Differential-planetary designs differ from planetary roller screws in the number of thread starts on the rollers and inside the nut. One common differential- planetary system assigns a zero lead to the rollers and nut — the members have circular grooves instead of threads. The resulting lead of the system is not positive and takes any value between zero (the lead of the nut) and the screw-shaft lead.
These units are difficult to position. The inherent design defect can be counteracted, but the resulting package includes several more parts which significantly reduce reliability.
Recirculating roller screws have a threaded shaft, a nut threaded to the same lead, and a set of grooved rollers. The rollers recirculate after each revolution within the nut because they travel axially by the amount of lead of the nut.
The concept allows for fine-thread leads that deliver high-precision positioning. However, the rollers are subject to high stresses in the recirculation area and cycling stresses from constant loading and unloading which eventually leads to failure.
Planetary Designs
Planetary roller screws have a threaded screw shaft with at least three starts of thread, a nut with same number of starts and identical thread profile, and a set of at least three rollers with a single thread start. For all practical purposes, most screws designs feature five or six starts of thread and 9 to 13 rollers.
The three basic elements are sized so the rollers freely roll inside the nut, follow a planetary path, and do not move axially. Therefore, there is no need for recirculation.
The screw shaft and nut thread have a modified triangular profile with a 90° angle. The roller-thread profile has the same angle, but with flanks shaped like those of a gear tooth. Thus, the thread generates a ball-shape contact area. For each lead of thread on the screw shaft, the number of ballshape contact surfaces equals the starts of thread.
The shape and number of contact points through which the load transfers from nut to the screw shaft dictate the dynamic load-carrying capacity Ca of a system.
This calculation, derived from the dynamic rating of ball bearings, defines the dynamic load a roller screw handles. Roller screws have a dynamic load-carrying capacity about three times greater than typical ball screws. And because predicted fatigue life of such components is proportional to the cube of the dynamic rating, roller screws can outlive ball screws of comparable size by a factor of 25 to 30.
In addition, because the rollers do not move axially or require recirculation, planetary roller screws can accelerate, rotate, and decelerate at rates far beyond the capabilities of ball screws. For instance, roller screws successfully operate at linear speeds of 1 m/sec and accelerate at rates exceeding 7,000 rad/sec2 — producing up to 3.5 g of linear acceleration, depending on the lead of the thread.
For equal helix angles, roller screws and ball screws have similar efficiencies. The exception is long-lead roller screws. Here, efficiency drops because the large angular gap between roller threads and the screw-shaft thread increases rolling friction. (For example, a screw with a 39-mm diameter and 25-mm lead has a helix angle of 11.53°, while the rollers have a helix angle -6.98°. The threads cross along an 18.51° angle.)
However, roller-screw inefficiency is in large part due to parasitic friction that occurs out of the load-transmitting path. As a result, efficiency does not decrease much with increasing load. In fact, efficiency tends to increase under high load as velocity increases. Thus, roller screws work best under heavy load at high speeds.
Roller-screw efficiency η is measured by the ratio of output torque to input torque,
System Losses
The difference between input and output torque represents system losses, dissipated as heat, vibration, noise, and wear.
The wear of surfaces in contact under load is the largest factor degrading performance, and also the most common failure mode. However, this is a slow process and planetary roller screws rarely wear out before 15 to 25 years of life. As a rule of thumb, normal backlash in a new unit is 0.03 mm maximum. An assembly can stand, in most cases, up to 0.10 to 0.15 mm of play with no noticeable degradation in operation. And because the process is slow, there is plenty of warning before complete failure.
While noise rarely presents serious danger, it may indicate an installation problem, most probably involving improper mounting. Lack of lubrication can also increase noise levels. Operating at one of the system’s critical speeds, caused by too high a rotational velocity, also creates vibration and noise.
Various tests comparing ball screws and planetary roller screws show the latter generally run quieter. For example, at 2,000 rpm a ball screw with 50 × 10 mm (diameter × lead) has an 82 to 83-dBA noise level, while a 48 × 10 roller screw operates at 68 dBA.
When a roller screw generates heat, the resulting temperature increase does not create any risk of sudden failure. However, it noticeably reduces the life of an assembly and must be prevented or controlled. Besides, heat has a detrimental effect on system accuracy. Planetary roller screws are manufactured with a lead accuracy of 10 to 35 μm/meter of thread. This is of the same magnitude as thermal expansion effects. For most steels used in roller screws, change in length due to temperature is found from:
where a = 100 × 10-7 mm/°C and ΔT = temperature change, °C.
One important problem that crops up when replacing hydraulic actuators by electromechanical actuators (EMAs) is heat management. In hydraulic systems, heat transfers to the hydraulic fluid and the main tank acts as a heat sink.
An EMA depends on lubricant flow to dissipate most heat. The following formula gives a rough estimate of the lubricant flow rate required to limit an actuator’s operating temperature.
Because this is a theoretical estimate, physical testing is recommended to verify results.
Preventing Misuse
While designers often tend to focus on wear and fatigue in looking at rollerscrew life, misuse is the most common cause of failure. For instance, misalignment is a major reason for premature failure because highly localized stresses accelerate wear.
Improper lubrication presents another problem. When possible, use grease with extreme-pressure additives. When oil is preferred for cooling use a minimum viscosity of 120 to 200 cSt at 20°C.
Dust and dirt on contacting surfaces also accelerate wear. Good practice dictates using screw-threaded wipers and covers or boots. Finally, shock loads can instantly destroy a system and must be avoided if at all possible. Minimize the effects of shock loads by adding a damping system integral to the planetary roller screw.
Unless subject to improper alignment or insufficient lubrication, wear should not be noticeable in the first years of planetary roller-screw operation. As surfaces erode, contact area under load increases, generating more friction and energy losses. For all practical purposes, surface erosion is slow under normal operating conditions and can be monitored by an increase in required torque or added play in the system. However, a screw assembly normally supplied with 0.03 mm maximum play can operate as long as maximum axial play does not exceed one-tenth the thread pitch.