One of the most important criteria when selecting a linear actuator is how far it needs to travel. The travel, or stroke length, can dictate the type of drive mechanisms that are practical (screw, belt, rack and pinion, or linear motor). But more importantly, specifying the wrong stroke or travel can leave you with an actuator that is too short to perform the task or process that’s required. On the other hand, an actuator that is unnecessarily long will take up valuable space and possibly interfere with other components in the system.
There are several methods that manufacturers use to specify the length of a linear actuator, and it’s critical to understand which one is being used for a given product, so you don’t end up with an actuator that has, for example, an overall length of 1500 mm, when you needed a stroke of 1500 mm.
This is the most basic of the length specifications. The stroke length, also referred to as nominal travel or working stroke, conventionally refers to the working travel—that is, the travel needed to carry out the process or application. The stroke length is typically the length between the limit switches.
Rod-type actuators are typically supplied in standard stroke lengths, such as 100, 150, 200 mm. While some linear actuators are also offered in standard stroke lengths, most manufacturers allow the customer to specify their exact stroke length in millimeter increments.
Also referred to as overtravel or safety stroke, this is an allowance that provides room for the actuator to coast to a stop if an emergency stop (e-stop) condition arises. Safety travel is intended to prevent the bearing table (and the load) from hitting the ends of the actuator.
Some actuators incorporate mechanical stops or bumpers at each end. However, unlike pneumatic actuators, electromechanical designs (those driven by a screw, belt, rack and pinion, or linear motor) aren’t designed to run into hard stops on a repetitive basis. The hard stops should be used only as a fail-safe mechanism, if needed, in the event of a runaway condition.
Safety travel is added to the maximum stroke length, and most manufacturers provide guidelines regarding the recommended safety travel for the actuator type. If no guideline is provided, a rule of thumb is to specify a distance equal to two turns of the motor on each end of the actuator, or four turns in total. For screw driven systems, this means four times the lead of the screw. For belt driven systems, this means four times the lead constant of the driving pulley. But if a gear reducer is used, each turn of the motor will yield a proportionally smaller amount of travel for the bearing table. For example, if a belt drive actuator has a pulley lead constant of 100mm, the recommended safety travel would be 400 mm (2 x 100 mm on each end of the actuator, or 4 x 100 mm total). But if a 5:1 gearbox is used, each turn of the motor will result in only 20 mm of travel, so the recommended safety travel would be 80 mm (4 x 20 mm).
Some manufacturers only want to know the total travel needed, from one end of the actuator to the other, and leave the allocation between working stroke and safety travel to the customer. This specification is used less commonly, but if the manufacturer does not provide a guideline or specification for safety travel, they may refer to the actuator’s travel in terms of total stroke.
Because many actuators are made from aluminum extrusions that are cut to length, the length of the housing (sometimes referred to as the body length) is a relatively common specification. The housing length is typically equal to the total stroke (working stroke plus safety travel), plus the length of the bearing table.
Although it doesn’t directly affect the actuator’s working travel, the overall length is useful for designing the actuator into the system. It is typically calculated by adding the hosing length (stroke length + safety travel + length of the bearing table) and the length of the end caps.
Other factors that affect actuator travel
External components, such as bellows, cover strips, gearboxes, motor mounts, and motors, can reduce an actuator’s stroke and total travel. When calculating the required stroke and total travel, be sure to check for interferences from components such as these.
Images credit: PBC Linear