By Wayne Greer, Engineering Manager and Project Sales, SKF USA
Monday, January 28, 2008
Turning To New Screw TechnologiesBy Wayne Greer, Engineering Manager and Project Sales, SKF USA
Although their role is a basic one, expanded application potential has translated to new ball and roller screw specifications and capabilities.Ball screws and roller screws can serve as one of the most efficient and cost-effective solutions for applications where precision linear drive systems are required. These anti-friction assemblies transform rotary action into smooth, accurate and reversible linear motion in transmitting axial loads. Applications for these technologies abound.
 The inner workings of an SKF roller screw, which the company feels can offer greater positioning accuracy. |
- Factory Automation: Planetary roller screws and ground ball screws for electromechanical actuation of spot welding, gluing, pick-and-place operations, riveting and clinching.
- Plastic Molding: Ball screws and planetary roller screws for injection molding, blow molding and thermoforming.
- Machine Tools: Ground and rolled ball screws and roller screws for broaching and stamping machines, CNC machining and routers.
- Medical and Healthcare: Miniature ball screws for medical imaging equipment, patient tables and blood analyzers.
While the particular application ultimately will influence specification, design engineers can begin the process by evaluating some basic, widely used screw selection parameters. That said, there are many operational factors that can influence drive screw type selection for a particular application. These factors might include (but are not limited to):
- Load profile.
- Linear and rotational speeds.
- Rates of acceleration.
- Cycle rate.
- Drive torque limits.
- Environmental issues.
- Required life.
- Lead accuracy.
- System stiffness.
Since all of these conditions can have a significant impact on performance, as much information as possible for an application should be pre-determined and communicated in advance of screw selection.
Screw Fundamentals
 Applications like this welding cell help demonstrate the improved accuracy and load tolerances of new ball and roller screws. |
Just to review, ball screws consist of a screw shaft, nut, balls and a re-circulation system. The screw shaft can be precision ground or use rolled threads that form a concave helical groove (acting as the inner race track).
A nut with internal grooves acts as the outer track. Circuits of precision steel balls circulate in the grooves between the screw shaft and nut. Depending on application demands, either a rotating screw shaft or nut can then move in a linear direction. Ball screws possess an inherent connection to higher efficiency functionality in converting about 80 percent of a motor's torque into thrust, which results in less mechanical wear and constant performance throughout the unit's lifespan.
Planetary roller screws feature threaded nuts, rollers and screw shafts. Their planetary roller motion eliminates any need for a recirculation system, and since the rollers do not need to re-circulate, the result is a design with fewer components, which typically translates to a longer life. The planetary timing mechanism enables high acceleration and proper functioning, even in harsh conditions, while multiple contact points on each roller can contribute to greater load-carrying capacity and, again, an extended operational lifespan.
Re-circulating roller screws incorporate a threaded shaft, a nut threaded to the same lead, and a set of grooved rollers. The rollers re-circulate after each revolution within the nut to allow for fine-lead threads (as small as 1 mm) that can deliver more accurate positioning, and maintain higher capacities.
Both types of roller screws can sustain static loads of up to 1,000 tons and dynamic loads of up to 200 tons. By most calculations this is three times more than comparably-sized ball screws.
Basic Selection Parameters
When making decisions involving ball screws or roller screws for an application, the first rule of thumb is to understand that a solution for one application may be inappropriate for another.
However, basic selection parameters apply to either screw technology and, by determining the following, the process can advance more quickly, cost-effectively and efficiently:
 When selecting a ball or roller screw driven system, make sure to consider the specifications required for the application, such as noise level, smoothness in operation, capabilities for repeatability and/or speed and acceleration. ADVERTISEMENT
|
- Dynamic Load Rating. This is used to compute fatigue life and represents the axial load, constant in magnitude and direction, in understanding when the nominal life (as defined by ISO) reaches 1 million revolutions.
- Nominal Fatigue Life. The nominal life is the number of revolutions (or the number of operating hours at a given constant speed) that the ball screw or roller screw can perform before the first sign of fatigue (flaking or spalling) appears on one of the rolling surfaces (screw, nut or rolling elements).
- Service Life. This represents the actual life before failure from fatigue or other factors, including inadequate lubrication and wear, wear of the recirculation system, corrosion, contamination or loss of the functional characteristics required by the application. Experience acquired with similar applications can help designers in selecting the proper ball screw or roller screw to obtain the required service life. Designers should also accommodate structural requirements, such as the strength of screw ends and nut attachments, due to the loads applied on these elements in service.
- Equivalent Dynamic Loads. The loads acting on the screw can be ascertained according to the laws of mechanics. The equivalent dynamic load is used to calculate the fatigue life. These loads can be a result of the external forces (power transmission, work, inertia forces, etc.), which must be calculated. Radial and moment loads should be determined early in the conceptual stage, since these forces can be detrimental to any screw's expected performance and service life.
- Static Load Carrying Capacity. Ball screws or roller screws should be reviewed for the their static load rating, especially if they will be subjected to continuous or intermittent shock loads while stationary or rotating at very low speed for short periods. (The permissible load can be determined by the permanent deformation caused by the load acting at the contact points.) The static load rating is defined by ISO standards as the purely axially and centrally applied static load that will create a total permanent deformation (rolling element plus thread surface) equal to 0.0001 of the diameter of the rolling element.
- Efficiency & Back-Driving. The efficiency of a ball screw or roller screw largely will depend on the geometry of the contact surfaces, their finish, and the helix angle of the thread, as well as the operating conditions of the screw (load, speed, lubrication, preload and alignment, among others). All should be considered early in the design process. Since both types of screws are reversible and may back-drive under almost all circumstances, designers should also consider that a brake mechanism (gear reducers or brake) will be necessary if back-driving must be avoided in an application.
- Preload. Preloaded nuts are subject to much less elastic deformation than non-preloaded nuts and, therefore, should be specified whenever the accuracy of positioning under load is important.
The parameters listed above are just a few to consider for proper selection of a ball or roller screw driven system. Other considerations can satisfy specific requirements, such as required noise level, smoothness in operation, capabilities for repeatability, or speed and acceleration. With all these possibilities, partnering with an experienced manufacturer as the design process gets under way can help sharpen focus for the selection process, and in meeting specific application challenges.