Don't be quick to think the belt is your limiting factor when sizing an actuator. There are other mechanical components to take into account.
Often, there is skepticism or concern about using timing belt driven actuators where a high amount of axial force is required. Our most popular timing belt driven actuator, the HPLA series, uses a polyurethane timing belt with reinforced wound steel cords. The steel cord gives the belt an increased tensile load rating. What does that mean?
To give you a better idea of how strong these belts are, the standard belt used on an 80 mm wide product (3 inches or 7.62 centimeters wide) has a tensile strength rating strong enough to hold an average size polar bear (~900 lbs) and a breaking strength rating strong enough to hold a Corvette.
Things to consider before assuming the belt is the limiting factor:
- Radial loading of your motor or gearbox if it is a direct mount
- Acceleration: high accelerations on bearings will cause the bearings to skid instead of roll which will create flat spots and failures
- Moment loading: be sure the carriage is properly sized to support normal or cantilevered loads
Other things to consider to prevent belt failure:
- Elongation or stretch: at its maximum tensile load, a steel reinforced 25AT10 timing belt will elongate 4 mm / m (or roughly 0.4%), which generally occurs very early in the machine installation/usage
- Slippage: an improperly tensioned belt can jump teeth easily with aggressive moves resulting in loss of position.
- Tracking: a belt that doesn't track true to the travel may move to one side and excessively load a pulley/bearing creating a failure mode
- Clamping: if a belt is not properly clamped or gripped, it will pull out of the clamping device with aggressive moves resulting in crashing units and loss of motion.
Timing belt driven actuators are a great way to lower the cost of a general motion system versus using a ball screw actuator. Knowing the facts will help you implement a successful motion solution with the best benefit payoffs to the application at hand.
Article contributed by Paul Koch, Applications Engineer, Parker Electromechanical Automation North America. Originally published on Parker Motion Blog June, 22, 2010
Related posts on actuators: