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Roller Screw Basics
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Why Consider Roller Screw Technology
A roller screw is a mechanism for converting rotary torque into linear motion, in a similar manner to acme screws or ball screws. But, unlike those devices, roller screws can carry heavy loads for thousands of hours in the most arduous conditions. This makes roller screws the ideal choice for demanding, continuous-duty applications.
The difference is in the roller screw's design for transmitting forces. Multiple threaded helical rollers are assembled in a planetary arrangement around a threaded shaft (shown left), which converts a motor's rotary motion into linear movement of the shaft or nut.
Designers have five basic choices when it comes to achieving controlled linear motion. The table below gives you a quick overview of what general advantages are associated with each technology.
Roller Screw vs. Hydraulic & Pneumatic -- Comparisons
In applications where high loads are anticipated or faster cycling is desired, Exlar's roller screw actuators provide an attractive alternative to the hydraulic or pneumatic options. With their vastly simplified controls, electro-mechanical units using roller screws have major advantages. They do not require a complex support system of valves, pumps, filters and sensors. Thus, Exlar units take up much less space and deliver extremely long working lives with virtually no maintenance. Hydraulic fluid leaks are non-existent. Noise levels are reduced significantly. Additionally, the flexibility of computer programmed positioning can be very desirable in many applications. |
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| xx |
Exlar
Roller Screws |
Acme
Screws |
Ball
Screws |
Hydraulic
cylinders |
Pneumatic
cylinders |
| Load ratings |
Very High |
High |
High |
Very High |
High |
| Lifetime |
Very long, many times greater than ball screw |
Very low, due to high friction and wear |
Moderate |
Can be long
with proper maintenance |
Can be long
with proper maintenance |
| Speed |
Very high |
Low |
Moderate |
Moderate |
Very high |
| Acceleration |
Very high |
Low |
Moderate |
Very high |
Very high |
| Electronic Positioning |
Easy |
Moderate |
Easy |
Difficult |
Very Difficult |
| Stiffness |
Very high |
Very high |
Moderate |
Very high |
Very low |
| Shock Loads |
Very high |
Very high |
Moderate |
Very high |
High |
| Relative Space Requirements |
Minimum |
Moderate |
Moderate |
High |
High |
| Friction |
Low |
High |
Low |
HIgh |
Moderate |
| Efficiency |
>90% |
approx 40% |
>90% |
<50% |
<50% |
| Installation |
Compatible with standard servo electronic controls |
User may have to engineer a motion/actuator interface |
Compatible with standard servo electornic controls |
Complex, requires
servo-valves, high pressure plumbing, filtering pumps, linear positioning and sensing |
Very complex, requires
servo-valves, plumbing, filtering, compressors, linear positioning and sensing |
| Maintenance |
Very low |
High due to poor wear characteristics |
Moderate |
Very high |
High |
| Environmental |
Minimal |
Minimal |
Minimal |
Hydraulic fluid leaks & disposal |
High noise levels |
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Roller vs Ball Screw Performance -- Comparisons
Loads and Stiffness: Due to design factors, the number of contact points in a ball screw is limited by the ball size. Exlar's planetary roller screw designs provide many more contact points than possible on comparably sized ball screws. Because this number of contact points is greater, roller screws have higher load carrying capacities, plus improved stiffness.
In practical terms, this means that typically an Exlar roller screw actuator takes up much less space to meet the designer's specified load rating.
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Travel Life: As you would expect, with their higher load capacities, roller screws deliver major advantages in working life. Usually measured in "Inches of Travel", the relative travel lives for roller and ball screws are displayed in the chart above. As you can see there, in a 2,000 average load application applied to a 1.2 inch (approximate) screw diameter with a 0.2 inch (approximate) lead, you can predict that the roller screw will have an expected service life that is 15 times greater. |
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Speeds: Typical ball screw speeds are limited to 2000 rpm and less, due to the interaction of the balls colliding with each as the race rotates. In contrast, the rollers in a roller screw are fixed in planetary fashion by journals at the ends of the nut and therefore do not have this limitation. Hence, roller screws can work at 5000 rpm and higher -- producing comparably higher linear travel rates.
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