Focus on Linear Motion: Avoiding the Vertical Load Mistake
By Warren Osak
When you're working with vertically oriented applications that employ rodless actuators, there are a few design considerations to keep in mind. All too often, motion control system designers fail to c...
November 1, 2003
By Warren Osak
When you’re working with vertically oriented applications that employ rodless actuators, there are a few design considerations to keep in mind. All too often, motion control system designers fail to correctly apply rodless actuators in vertical applications. This mistake is common, but a better understanding of the technology will help you avoid it.
Rodless actuators contain a bearing structure that guides and supports the load, while a drive system (e.g., an Acme screw, ball screw or belt) moves the load. The load is attached to both a carriage that’s guided by the bearing structure and the drive system. An attached electric motor drives one end of the drive system.
Rodless actuators are great for saving design time and are offered in a wide range of load and thrust capacities with a selection of different motor types (e.g., brushed, hybrid steppers or brushless servos).
Belts vs. screws
Belt-driven actuators have certain advantages over screw-driven systems, as they can run at higher velocities and for longer travel distances. Screw-driven actuators are limited by the screw’s critical speed, which is the natural frequency where harmonic vibrations occur as the screw is spinning. This critical speed limitation can begin imposing actuator limits starting as low as 18 inches.
The critical speed is a function of both the screw’s diameter and its unsupported length. However, this limitation doesn’t affect belt-driven actuators, making them a good choice for high-speed, long-stroke applications.
Ball screws have recirculating ball bearings along the screw. The bearing’s point-to-point contact efficiently converts rotary motion to linear motion. Ball screw-driven rodless actuators are better than belt-driven actuators for higher load capacity. This is because the thrust capacity’s limiting factor is the column load (the steel screw’s axial strength) rather than the carriage belt’s tension limit (which can have steel tensile members).
As a comparison, a standard series belt drive unit’s thrust capacity is 300 lb, whereas a screw-driven unit of the same size can have a maximum thrust of 700 lb.
One of the more common mechanical mistakes in motion control occurs when designing ball screw- or belt-driven actuators into vertical applications.
Typically, both systems have a mechanical efficiency of 90 per cent. In the case of a power loss, however, the motor has no holding torque, which causes the load to come crashing downward.
There’s some inherent system friction that can hold light loads, but rodless actuators will typically back drive with 15-20 lb of force. Resistant system forces that contribute to this back drive are the mechanical inefficiencies (including opposing applied forces on the load); static friction in the bearings and the drive system; any reduction (timing belt or gear) inefficiency; and the motor’s detent torque when it isn’t powered).
The pitch of a screw-driven system also contributes — i.e., a 2-revolutions/ inch (rev/in.) ball screw has a lower back drive than a 5-rev/in. screw. This is because the relationship of frictional torque to an axial holding force is F = 2*Tp, where T is the frictional torque, and p is the ball screw’s pitch.
If the power fails, these system inefficiencies will resist the load’s inclination to back drive, but if the load is heavier than these back drive forces, it will fall uncontrollably.
There are a few solutions to prevent this. First, you can place a brake on the motor or ball screw. Most brakes are fail-safe, meaning power needs to be supplied to release the brake from the load (or, conversely, the brake automatically engages the load in the event of a power loss). Programmable controls have configurable outputs preprogrammed for brakes that automatically release and engage the brake prior to a motion command or in case of a fault. The time it takes to engage the load is called the set time. The set times for brakes are typically on the order of 100 milliseconds.
For ball screw-driven systems, one option is changing the drive system to Acme screws. Acme screws have a solid nut made of plastic or bronze that engages the screw threads, much like an ordinary nut and bolt, for several revolutions.
Acme screws, sometimes referred to as lead screws, have lower mechanical inefficiency than ball types (in the order of 60 per cent). This increased frictional force is self-locking in a power loss situation, possessing a back drive force of several hundred pounds. For instance, a typical rodless actuator, with a 5/8-in. diameter, 5 rev/in. Acme screw, has a back drive of 300 lb.
There are drawbacks, however. The heat built up by excessive friction limits the actuator’s duty cycle, preventing the actuator from running constantly. Furthermore, this system can’t transmit the motor’s torque as efficiently, and thus thrust decreases when compared with the more efficient ball screw/brake combination.
Counterweighting or load braking are both options. Either would be good for applications with a human risk factor. If a belt snaps on a belt-driven system, a brake on the motor isn’t going to help, and the load is going to crash! This is where either braking or counterweighting the load or drive system would be the clear choice.
Counterweighting the load acts much like a teeter-totter, balancing the load at all times, even in case of a power loss. This approach’s disadvantage is that it increases the system’s inertial mass; consequently, the systems may exhibit excessive overshoot or sluggish performance.
Sometimes this counterweight isn’t as heavy as the load, so its function is to maintain a controlled fall. This configuration is used to prevent mechanical damage from hard crashes where there would be no human risk. Counterweighted systems are typically designed by machine builders and added to rodless actuators after being installed into the machine.
Another option, similar to counterweighting, is directly braking the load (e.g., using linear brakes or ratcheting systems). Either programmable drives or motion controllers can engage such devices. However, these systems are rather design-intensive and would be cost-effective only for large-volume machines or if called for in special applications, such as those involving a human risk factor.
To prevent equipment-damaging hard stops in short stroke-length applications, another alternative is to use a spring device to soften any sort of hard stop. Examples include springs and rubber stops. (High-speed horizontal applications may also employ these devices in case of runaway conditions.) They’re designed to absorb and store some of the energy rather than allowing mechanical parts to endure the shock of a crash.
If you’re designing rodless actuators into vertically oriented applications, you’ve got a number of options available. Finding yourself with an unbraked load, however, isn’t one of them. Considering different application factors will help you avoid this common mistake and ensure you’re focused on success — both in machine design and in motion control.
Warren Osak is with Electromate Industrial Sales Ltd., Woodbridge, Ont. For more information, visit www.electromate.com or call 1-877-737-8698.