Unlike piston rod actuators, where a push rod extends unsupported from the housing, rodless actuators use a slide (or carriage) on which a rail-supported adjustment mechanism moves the load back and forth. To achieve an optimal combination of efficiency, reliability and performance, several variables must be carefully balanced.

Similar basic principles apply: Rodless actuators are primarily differentiated by the length, width and height of their profiles. Common dimensions range from 40 x 40 to 120 x 120 millimeters. In order to exploit the potential advantages within this spectrum, the required stroke length, maximum speed, dynamic carriage load and bearing requirements must be precisely analyzed. Only on the basis of this analysis is the motor selected, and the attachment devices and required control functions determined. The central components in this selection process are the adjustment mechanisms and the guide systems, supplemented by the specification of the power factor, the motors and sensors as well as the other accessories.

Selection of adjustment mechanisms
In rodless actuators, sliding or ball screw drives or timing belts are usually used.

Lead screws: These solutions, also known as lead screws or trapezoidal screws, offer high rigidity and adjustment force in a compact design. The power transmission between the screw and nut is sliding, which results in a high friction factor and rather low efficiency. On the other hand, this technology offers the advantage of a certain degree of self-locking in the rest position. The service life of the actuator is determined by the play caused by wear between the nut and spindle. However, there are now manufacturers who equip their actuators with self-lubricating plastic nuts in order to extend the service life.

The ratio between pressure load and speed (PV) is another factor that determines the load rating and permissible speed of lead screws. Pressure corresponds to displacement force - so if the displacement force increases, the speed decreases and vice versa. Actuators with a lead screw can reach stroke lengths of three meters, but are usually found in short-stroke applications with lower loads.

Ball screw actuators: Ball screw actuators are more robust and are also better suited to most industrial applications. For example, a ball screw actuator on a 120-millimeter extruded profile with a precision-rolled ball screw with a diameter of 32 millimeters and a lead of 20 millimeters offers an adjustment force of 12,000 newtons and a maximum adjustment speed of one meter per second at a drive speed of 3,000 revolutions per minute. The same actuator and spindle diameter with a pitch of 40 millimetres, on the other hand, offers an adjustment speed of two meters per second and an adjustment force of 8,000 newtons. A typical combination of precision-rolled ball screw and ball screw nut achieves a position repeatability of plus/minus 0.01 millimeters or less.

Rodless actuators offer a wide range of options depending on the requirements of the system design. © Thomson

Actuators with a rolled ball screw therefore offer a cost-effective solution for applications with a stroke of up to three meters. Ball screw actuators can be selected to optimize the power density, i.e. to achieve the highest possible adjustment force in a small size. The nominal life L10 can be accurately predicted as the ball screw nut is essentially a ball bearing subject to the same ISO calculation. The length of the screw has an effect on the load rating due to the buckling load capacity, just as the permissible speed is generally limited by the vibration development.

Two other factors that contribute to the load rating of a rodless actuator with ball screw are the design of the ball screw nut and the slide length. A longer or double ball screw nut increases the adjustment force, while a longer slide increases the length or spacing of the bearings and the moment load capacity. For example, Thomson Industries offers a ball bearing guided ball screw actuator with single or double nut. The single nut version of this unit with an 80 by 80 millimeter profile has a 200 millimeter long slide, an adjustment force (FX) of 3,500 newtons and a pitch torque (My) of 180 newton meters. The corresponding version with double nut has a 280 millimeter long slide, an adjustment force (FX) of 5,000 Newton and a pitching torque (My) of 300 Newton meters. In addition, the double nut itself has greater rigidity, slightly better repeat accuracy and less axial play.

Adjustment mechanisms with control belts: These actuators are also robust and are suitable for most industrial applications. Actuators with tension-resistant timing belts can withstand high adjustment forces, adjustment speeds of up to ten meters per second and are practically unlimited in length. For applications that require stroke lengths of more than three meters, they are the more cost-effective alternative to actuators with rolled ball screws. In addition, timing belt actuators are particularly clean and at the same time insensitive to dirt.

Selection of guide systems
Another critical success factor for rodless actuators is the specification of the guide systems that support the carriage, but also all the forces that act on the load to be moved. You can choose between plain bearings, ball bearings, cam rollers and/or wheels.

Plain bearings: Plain bearings have a higher friction factor than the other solutions, but on the other hand often manage without lubrication and withstand dirt or moisture. They also dampen vibrations, run quietly and tolerate short strokes with fast working cycles. Plain bearings or guides run either directly on the extruded aluminium profile or on rails embedded in the profile made of various materials such as hardened steel, stainless steel or anodized aluminium. The plain bearing bushes can also be made of different materials, such as polymer, PTFE and numerous low-friction plastics.

Ball bearing guides: These variants consist of circumferential ball bearing bushes that are mounted on the carriage and run on rails made of hardened steel, which in turn are screwed onto the profile or onto steel inserts. They can either be designed as a single profile rail for a more compact actuator or as a more stable double rail. Ball guides offer high accuracy and load rating and support medium speeds.

Wheels and cam rollers: These simple and economical solutions also run on rails made of hardened steel that are embedded in the profile. They offer high load ratings and speeds with medium accuracy. The encapsulated ball bearings are maintenance-free and dirt-resistant.

A motorized lead screw drive offers increased torque density and improved battery life. © Thomson

Power factor
The power factor is a decisive variable when selecting a rodless actuator. In a typical working cycle, an actuator accelerates, runs and decelerates at least once per direction of movement. However, there can also be many intermediate stops over the stroke. The moment loads indicate the force required to accelerate the driven component and its position relative to the carriage. Even with driven components with their own guide systems, pitch and yaw moments act on the carriage.

Motor versions
Servo or stepper motors are mainly used to drive rodless actuators. Standard DC and AC motors are also used, but their duty cycle is limited to the number of starts per hour that the motor can perform without damage. Actuator manufacturers typically offer metric and NEMA mounting brackets with or without the motor attached. The size of the motor is ultimately determined by the speed and torque requirements.

Automatic selection process
Linear Motioneering is an online tool that Thomson offers for the design and selection of linear components, including linear actuators. Users enter information such as ambient conditions, repeatability, stroke, travel distance, travel time, duty cycle, alignment, loads and forces, from which the tool determines a customized actuator solution.

Let's take the design of a packaging machine as an example, which requires a horizontal axis with a stroke length of 1,500 millimetres to pick up products and place them in boxes. After entering the stroke length together with other application-related parameters, such as the mass connected to the carriage, any lateral offset from the carriage to the center of the mass and requirements for the working clearance, the best and most cost-effective solutions are output

Once the basic actuator model has been selected, the tool also helps with fitting sensors, mounting brackets and a motor mounting kit, including screws and couplings. The result is a complete order code including all price details. As if that were not enough, the tool calculates a complete motion profile with continuous and peak torques, maximum adjustment speeds and expected running performance. as