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Roller Screw
High-Force
Linear Actuators

Choose the right screw technology for your high-force electric linear actuator.

Ball, Planetary & Inverted Roller Screws

How to select the correct screw choice for high force linear actuators:
Standard Planetary Roller Screws, Inverted Roller Screws, or Ball Screws.

There are several screw technology options to consider when specifying high force electric linear actuators – standard planetary roller screws, inverted roller screws and ball screws. Each technology has its advantages and disadvantages when compared to one another.

To select the best option, it is critical to let the application specifications be the guide. Some companies only sell ball screw linear actuators and some companies only sell roller screw linear actuators. Take caution when evaluating “there is only one technology” type vendors. Every application is different and both technologies (roller screw and ball screws) have their place. By evaluating the application details and both technologies, it will ensure you optimize performance and cost.

Tolomatic manufacturers both roller screw and ball screw high force electric linear actuators. Tolomatic's online electric actuator sizing software guides engineers through the application specifications including force, speed, dwells, life estimates and other factors in order to make the correct choice.

Standard Planetary roller screws offer the highest dynamic load rating for the longest life of the three technologies. Both roller screw technologies are good choices for repeated stress applications in same area of actuator stroke. Applications such as pressing, inserting, riveting are applications with repeated stress. Roller screws have significant increase in contact area which allows for longer life and exerts higher forces in the same package size versus ball screws. However, this increased contact area also creates more heat with the same amount of work. Ball screws, because they have less contact points, are a little more efficient in heat management than roller screws which allows them to operate cooler in high duty cycle and high speed applications. Heat management is a major factor in how well lubricants hold up over time and whether the actuator/screw choice can last as long as expected. There are other factors in each screw technology deployed in an actuator that determines their advantages and disadvantages. See the comparison table below.

Comparisions of Electric Screw Cylinders,
Hydraulic & Pneumatic Cylinders

  ELECTRIC CYLINDER HYDRAULIC CYLINDER PNEUMATIC CYLINDER
Specification Standard Roller Screw Inverted Roller Screw Ball Screw
Load Rating (DLR) Very High High Medium-High Very High High
Actuator Life Very High High Medium High/Varied Medium/Varied
Peak Speed High High High Very High Very High
Peak Acceleration High High High Very High Medium
Electronic Positioning Easy-Servo System Easy-Servo System Easy-Servo System Extra Servo-Hydraulics Extra Servo-Hydraulics
Stiffness Very High Very High Medium Very High Very Low
Shock Loads High High Medium-Low Very High Very High
Backdrive Medium-Low Medium-Low Low Very High,
if valve closed
Very High,
if valve closed
System Efficiency ~75% ~75% ~80% ~40%-55% ~10-25%
Energy Consumption Low Low Lowest Medium-High High
Environmental Minimal Minimal Minimal Fluid Leaks/Disposal High Noise
Extreme Temperature Very Tolerant Very Tolerant Very Tolerant Seal Failure, Sluggish Operation Seal Failure, Sluggish Operation
Maintenance Low Low Low Very High High
Stroke Length Long 457mm or less Long Long Long

 

Tolomatic Planetary Roller Screws

Planetary (standard) roller screw
linear actuators

Roller screws (also called planetary roller screws) have triangular-shaped, precision-ground threads that match multiple precision-ground threaded rollers in the nut. These rolling elements transmit force very effectively. Roller screw components are designed with a fine pitch, providing more points of contact and a larger contact radius. The result is less stress per point of contact.

Tolomatic Roller Screw/Nut Diagram

Roller screws have very high force transmission capabilities since the rollers have significant contact with the screw threads. They can deliver high force, operate at high speeds, are long-lasting and require little maintenance. 

The use of planetary roller screws is allowing engineers to specify electric actuators in applications that were once the sole province of hydraulic cylinders.

Tolomatic manufacturers standard roller screws for its actuators. These screws are case (surface) hardened before precision grinding, resulting in very deep case hardness and high Dynamic Load Rating (DLR*.)

Tolomatic offers a range of high-force electric linear actuators, all with long-lasting standard planetary roller screws:

ServoWeld Resistance Spot Welding Actuators

  • High force efficiency
  • Multiple designs
  • Best-in-class performance
Tolomatic High Force RSA-HT Electric Actuators

RSA-HT heavy duty electric rod actuators

  • 4 body sizes
  • Strokes from 18 to 60 inches (457 to 1,524 mm)
  • Maximum force/thrust up to 13,039 lbf (58,001 N)
Tolomatic Extreme Force RSX Electric Rod Actuator

RSX extreme force electric rod actuator

  • RSX-096 is capabable of 30,000 lbf (133.5 kN) 
  • New sizes coming soon with forces up to 50,000 lbf (222.5 kN)
Tolomatic IMA Servo Linear Actuators

 IMA integrated servo motor actuator 

  • Ball and Roller Screw selections
  • Forces to 6875 lbs (30.6 kN)
  • Integrated servo motor design

IMA food grade linear actuator

  • Ball and Roller screw selections
  • Food grade white expoxy paint
  • Forces to 6875 lbs (30.6 kN)
Tolomatic ERD Hygienic Electric Rod Actuators

ERD hygienic electric actuator

  • USDA and 3A approved
  • Clean-in-place
  • Force/thrust capabilities up to 7,868 lbf (35 kN)
Inverted Roller Screw Design

Inverted roller screw linear actuators

An inverted roller screw operates much the same as a standard roller screw except the functions of the nut and screw are reversed. The rollers move inside the nut.

Most inverted roller screws used in linear actuators are produced using a process other than grinding to inexpensively create threads inside the nut. The hardening process is performed after the nut is machined and results in a much shallower case hardness depth and softer threads than those of standard roller screws. This leads to a significantly lower DLR* (lower life) and more challenges with maintaining lubrication.

Standard Planetary Roller Screw vs.
Inverted Roller Screw

SPECIFICATION STANDARD INVERTED
Manufacturing Method Precision Ground Mixed
Case Hardness Depth ~1.0mm (~100x greater) ~0.01mm
Screw DLR Size 3 12,050 lbf (53,6 kN) ~5,800 lbf (25.8 kN)
Size 4 16,479 lbf (73.3 kN) ~8,300 lbf (36.9 kN0
Lubrication Maintenance NO removal or
disassembly required
Must remove and disassemble
the front of actuator

 

The chart above reflects integrated servo actuators with standard and inverted screw designs. The DLRs listed here are selected from like-sized product.

By utilizing the inverted design in an actuator, the actuator may be a little more compact than an actuator with a standard roller screw, but this comes at a sacrifice of lower DLR (lower life). Because of this, Tolomatic only supplies actuators with standard planetary roller screws to maximize service life.

Rockford Ball Screw

Ball screw linear actuators

Ball screws get their name from the re-circulating ball bearings that fit between arch-shaped screw threads and corresponding threads in the nut. The ball bearings transmit force and relative motion efficiently as they roll through one or more circuits in the nut.

Ball screw actuators have higher thrust capabilities, longer service lives and higher efficiency than those with acme screw systems, but they can’t match the performance of roller screw actuators. Ball screw actuators can be back-driven and can be noisy. They’re ideal for applications that require high duty cycles, moderately high thrust and moderate speeds. They tend to be reasonably priced, making them popular in many applications.

When it comes to comparing roller screw vs ball screw of similar size and lead, a ball screw’s ball bearings have fewer points of contact than roller screws as shown in the diagram below..

Tolomatic Ball Nut Diagram

 

 

 

 

 

 

 

This, plus a design that allows the bearings to contact each other, limits the ball screw’s DLR*, leading to lower forces and shorter life.

SPECIFICATION ROLLER SCREW BALL SCREW
Dynamic Load Rating Very High Medium
Screw Life Very Long, Many times greater than ball screw Moderate
Shock Loads Very High Moderate
Relative Space Required Minimal Moderate
Maintenance None to Minimal Minimal

 

For most applications that require high force, repetitive cycles and long expected life, Tolomatic will recommend a roller screw linear actuator. However, if the force is lower and high continuous speeds are present in the application, Tolomatic may recommend a ball screw actuator.

What is dynamic load rating (DLR)?

Tolomatic for roller screw, high force linear actuators

Choosing to work with Tolomatic for high force linear actuators means understanding the application specifics and choosing the right technology for the job. Tolomatic does not take a “one technology” approach to solving customer needs and can provide industry best sizing & selection, application advice, quality, pricing, delivery (15 days) and after market service (free, quick evaluation) on both roller screw and ball screw linear actuators for high force applications.

Start learning now with our guide to actuator screw technology, Which Screw? Picking the right technology. Or, download our e-Book on A Resource on Electric Linear Actuators: What a machine builder needs to know. For intelligent, helpful advice on linear actuator selection, contact us or your local Tolomatic distributor.