When it comes to linear motion engineering (or any technical field, for that matter), nothing can slow down or derail a project more than misunderstanding the parameters or data relevant to the design. While there are some general technical terms we all know, there are others that some may have not encountered or needed to use until selecting an electromechanical actuator or designing a linear motion system. Below we offer definitions to some relevant terms that are key to the design of linear motion solutions.
Lead accuracy is a measure of how accurate the lead of a ball screw is. For a ball screw with a lead of 25 mm, the screw should move the nut 25 mm per each revolution. In reality, there will be a deviation between the expected traveling distance and what is actually achieved. The deviation for a ball screw is typically 0.05 mm per 300 mm of stroke, but some higher accuracy screw variations are available that can offer a decreased deviation.
Positioning accuracy is the error between the expected and actual position and is the sum of all factors that will reduce the accuracy (i.e. repeatability, backlash, resolution, screw/belt accuracy, and the accuracy of the motor, drive, and motion control system). Some of these factors, such as backlash and lead accuracy, can sometimes be compensated for in the software of the motion control system being used.
Repeatability is the ability for a positioning system to return to a location when approaching from the same distance, at the same speed and deceleration rate. Some of the factors that affect the repeatability are the angular repeatability of the motor, drive and motion control system, system friction and changes in load, speed, and deceleration.
Note: Electromechanical Actuators are proven to be more accurate and repeatable than hydraulic or pneumatic actuators. Learn more about these advantages by downloading our free guide, Accuracy and Repeatability in Linear Actuators: Why Electromechanical Systems Outperform Pneumatics and Hydraulics.
Backlash is the stack up of tolerances (play) within the lead screw/belt transmission assembly and gearing which creates a dead band when changing directions. The result is that the motor can rotate some before any motion can be seen by the actuator when reversing the direction of the motor rotation. The backlash present in an actuator varies depending on the liner motion system model.
All screws have a critical speed where the screw starts to vibrate and eventually bends or warps the screw. The exact limit is a function of the length of the screw and the speed of rotation. For some units, this means that the allowed maximum speed found in the performance specifications can be higher than the critical speed when the stroke exceeds a certain distance. In this case, either the speed must be reduced to the critical speed, the amount of stroke must be reduced, or you must use a screw support option if the unit in question allows this. Otherwise, you must select another unit that can manage the speed at the desired stroke.
Idle torque is the torque needed to start the motion of an actuator with no load applied by rotating the drive shaft. The idle torque will vary according to the input speed, screw type and lead, and unit type.
A ball guide consists of a ball rail and a ball bushing. The ball rail is made of hardened steel and typically runs along the inside of the actuator profile. The ball bushing is attached to the carriage/drive plate of the unit and contains ball bearings that roll against the rail. The balls in the bushing can be recirculating or have fixed ball positions depending on the type of ball guide. The recirculating type has a longer life and better load capability while the fixed type typically is much smaller in dimensions. Hunt Valve Actuator Division uses three major types of ball guides in its linear motion systems. Either the compact single rail type with recirculating ball bushing (A), the stronger double rail type also with recirculating ball bushings (B), or the fixed ball position ball bushings type (not shown) which require very little space and are typically used in the smallest units. Ball guides offer high accuracy, high load support and medium level capable speeds.
A slide guide consists of a guide attached to the inside of the profile and a slide bushing attached to the carriage/drive plate. The guide can be made of different materials (e.g. polished hardened steel, anodized aluminum) while the bushing is made of a polymer material. There are two types of bushings: fixed and prism. Prism bushings can move in relation to the guide which results in longer life and higher load capabilities. Slide bushings are silent, simple, reliable, and robust and can be used in dirty and dusty environments. They are also resistant to shock loads, have a long-life expectancy, and require little or no maintenance.
A wheel guide consists of ball bearing wheels that run on a hardened steel rail. Wheel guides are a simple and robust guiding method offering high speeds, high loads and medium accuracy.
Screw supports allow screw driven units to travel at high speed even when the stroke becomes longer. The supports reduce the unsupported length of the screw that otherwise would be subjected to vibrations and screw whip. Screw supports come in single (one screw support on each side of the carriage) or double (two supports on each side) versions. Screw support units will have a slightly shorter stroke for a given overall length as compared to a unit without screw supports.
For a full list of technical terms, download our Rodless Actuator Catalog. Aside from highlighting the rodless actuators Hunt Valve offers and their specifications, it includes a glossary that you’ll no doubt find helpful. Hunt Valve Actuator Division specializes in designing electromechanical linear motion systems for a wide range of applications. From off-the-shelf products to completely customized solutions, we can help with your unique needs. Contact us or request a quote today.