eDrive Magazine – Motors and Drives Conference

eDrive Magazine

Belt or Screw Drive Actuator: Which One to Choose?

By Glen Michalske, Glen Michalske Business Development


Figure 1: Belt drive and screw drive actuators, courtesy of PBC Linear.

Deciding whether to use a screw or belt drive actuator can be
a difficult decision. Both types of actuators generally feature antifriction ball bearing guidance systems that provide for predictable, long, reliable travel life, even at 100 percent duty cycle and excellent moment-load capacity. The ball bearing systems can be further delineated by reticulating linear ball bearing, linear guide or a cam follower riding on a flat or V hardened rail.

There are a number of manufacturers that also use sliding polymer guides, commonly called plain bearings, in lieu of antifriction bearings for cost advantages. Antifriction bearings stay accurate until they fail, whereas the sliding polymer
guides start wearing away over time. The life of the polymer bearing system is directly attributable to the PV values of the polymer. If the load is light, these types of actuators are very satisfactory in applications and are capable of high speeds and accelerations under light loading with minimal cantilever loads. Some manufacturers even build an adjustment system into the bearing housing to adjust for wear in the system. Because there are no ball bearings in the system, they tend to be quieter than an antifriction system.

Figure 2: Drives with built-in motion controller, courtesy of Fastech.

Figure 3: Lead screw plane bearing actuator, courtesy of PBC Linear.

The advantage of polymer bearing guides is that they are generally manufactured with a generous
amount of PTFE or have specially formulated polymers that alleviate the lubrication issues with the guidance bearings. However, if the actuator has a ball screw for a drive, it will require lubrication. Cam follower, reticulating ball bearing and linear guide type systems can be preloaded to ensure actuator stability and repeatability and have a lower coefficient of friction than plane bearings. They are also capable of taking more load at high speeds.

Ball screw and belt drive actuators can be coupled to high-performance servo systems to ensure precise positioning. Many units feature magnetically sealed stainless covers to protect against the egress of contamination. They also share convenient t-slot mounting, limit switches, encoders,
multiple carriages and motor and gearbox mounting flanges.

Drive technology selection
Selecting the right drive technology comes down to speed and travel length with considerations
toward three things: accuracy, rigidity and repeatability.

Ball-screw-driven units offer a stiffer system and are generally more accurate, depending on the type
of ball screw in use. Typically, the actuators employ a class 10 screw (.0004/ft accumulative), and class 5 screws are also available with a non-preloaded ball nut. Optional preloaded ball nuts are available. The limitations to screw-powered units are travel length and speed. Screw-driven units succumb to a phenomenon called critical speed, which is a wave harmonic that distorts the screw when the screw rotates too fast in relation to its travel length. Actuator manufacturers provide critical speed charts that should be adhered to when selecting the actuator.

Screws like to run in a straight line, and it is important to monitor the distance between mounting surfaces with regard to load, especially in a horizontal application. Too much droop or deflection in the actuator body will affect the critical speed and column strength ratings and will cause damage to the unit. Consequently, it is also important to ensure the travel length accommodates the column loading on the screw. Lead screw

Figure 4: Electric linear actuators are quickly replacing pneumatic and hydraulic actuators in applications that require control, accuracy and high performance. Industries such as packaging, pharmaceutical, food & beverage, semiconductor and factory automation are good candidates.

(Acme/Trapezoidal) equipped units are available, and the above rules also apply to these. Lead screws are quiet and generally require no lubrication. Some, depending on the lead, are self-braking (5 mm or lower) alternatives to ball screws. However, their life is not predictable, and their natural wear may affect future system accuracy. Preloaded versions are available to improve repeatability. Customers tend to use closed-loop servo systems with high-accuracy ball screw or preloaded lead screw systems.

Belt-driven actuators provide more flexibility in the mounting surfaces. They are available in up to 4
m of continuous length and may be butt-jointed for longer lengths. To obtain the optimum load and
speed performance, they often have to be inertia-matched by an external gearbox coupled before
the motor. They also should employ a brake in vertical applications. In any vertical application,
including ball screws, it is a best practice to include a brake. Belt drive units go fast, as fast as 5 m/sec, and, due to the steel-reinforced belt, are very repeatable and are less likely to be affected by actuator droop or twist. The low carriage mass coupled to a steel-reinforced timing belt provides high acceleration with sustained velocity.

Acceleration at 20 m/s2 and speeds as high as 8 m/s can achieve performance comparable with
linear motors at a fraction of the cost.

Both screw and belt-driven units coupled with servo motors and controls can provide a compact,
high-performance, accurate linear motion solution at a competitive cost. Many applications call for
belt- and screw-driven units used in combination. Belt drive units normally have their drive sticking
off the side, whereas screw-driven actuators are often in line or may have a J-style motor mount.