When choosing the right size ballscrew, engineers often limit their analysis to just lead accuracy or load capacity. These are important factors in selection, but there are others. Overall ballscrew performance, including size, weight, and power consumption are also crucial in selecting the right size ballscrew. Failure to consider all factors is the most common design specification error made with miniature ballscrews.
Miniature ballscrews typically have nominal diameters ranging from 3 to 16 mm. They are selected for their smooth operation with constant idling torque, rather than for their high load capacity.
These ballscrews are mostly used in devices and instruments that transport small objects, such as semiconductor devices or microscopic items. Because these instruments are sensitive to any motion resistance, friction of all their mechanical components must be low and constant. One of the reasons engineers choose miniature ballscrews, particularly those with diameters from 3 to 5 mm, is because they can deliver the required smooth motion.
Along with smoothness and lead accuracy, engineers should also consider the type of thread, any effects from the application’s required linear speed, style of ballscrew nut, and determine the need for backlash.
Accuracy. Ballscrew accuracy is a combined function of the controller-motor- ballscrew-bearing-encoder system. There are several grades, (see the box, “Grading accuracy.”)
The accuracy of the lead length (the distance between threads on a ballscrew) is one of the main components of ballscrew accuracy. As a general rule, the smaller the ballscrew, the higher the lead accuracy necessary to ensure proper function. The optimum mechanical drive is often a ballscrew with lead fine enough to provide the desired resolution but coarse enough to provide sufficient thrust. Miniature ballscrews typically have a high lead accuracy, from 0.5 to 5 mm. If preloaded linear bearings are used as guideway elements, the optimum lead for maximum repeatability ranges between 2 and 4 mm.
A surface scanning instrument, for example, may use a ballscrew with accuracy class zero (maximum lead error of 4 mm per 300 mm thread length), even though it actuates an auxiliary operation rather than the main measuring function of the instrument.
Small leads help achieve fine positioning. However, the small ball diameters used with these leads have less load capacity and lower allowable preload, which reduces overall stiffness. This is because preload can eliminate backlash and increase overall ballscrew stiffness.
Engineers should choose the lead length based on the required maximum velocity. The two main constraints to consider are nut speed (based on the ballscrew nut style) and critical screw speed. Manufacturer’s publish the maximum nut speed in their specifications. However, you will find that a nut with internal recirculation often has a higher value than one with an external tube. You can calculate the critical speed of the spinning thread from the supportbearing method used (fixed-fixed, fixedsupported, and so on), the nominal diameter, and the overall length.
Load. As mentioned earlier, ballscrews with diameters of less than 8 mm are usually chosen for their smooth operation or to fit tight space constraints. Ballscrews from 8 to 16 mm in diameter are selected according to load carrying capacities and lead. Engineers can use the formulas for static and dynamic-load capacities available in manufacturers catalogs to select the proper miniature ballscrew size.
Threads. Threads on the 3 to 8 mm diameter ballscrews are usually precision ground. As the ball sizes get smaller, the sensitivity of the miniature ballscrew to machining tolerances increases. Ball diameters, restricted by lead and diameter of the screw, are typically equal to or less than 1 mm. Thus, only precision grinding can ensure proper operation of the ball circuit.
The threads of ballscrews over 8 mm in diameter may be ground or rolled.
Linear speeds. An important design guideline, often overlooked, is that high linear speeds (about 2,000 rpm) affect the required minimum thread accuracy. High linear velocity often involves high accelerations, usually more than 1,500 rad/sec2. With such accelerations, balls may skid rather than roll. This will cause the lubrication film to fail, which will lead to excess heat generation and subsequent failure of the ballscrew. Preloading the nut and special lubrication can avoid this condition.
In turn, though, the need for a preloaded nut might call for a screw of higher accuracy grade than otherwise required.
At high speeds, vibration is a concern. Thus, the straightness of the screw and concentricity of the bearing points and ballnut races become important factors. Any of which may point to the need for a ballscrew with much better lead accuracy than initially specified.
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Ball return choices. As with larger ballscrews, miniature ballscrew nuts are available in two styles: external-return tube or internal ball-deflector. Ballscrews with diameters less than 8 mm have ball nuts that exclusively use an internal ball deflector system. These systems allow balls as small as 0.6 mm to pass through and recirculate without jamming. Tube-type nuts generally lack the ability to smoothly handle these ball sizes. Also, internal return nuts are smaller than tube type.
The 8 to 16 mm ballscrews may use external tube type nuts or internal ball return systems.
Backlash. Miniature ballscrew nuts are available with or without backlash. Even if the application does not call for it, it is often necessary to reduce or eliminate backlash to ensure smooth operation and eliminate possible shock loads.
The methods used to eliminate backlash are oversize balls, preloaded single nuts, or spring-preloaded double nuts. Use of oversize balls provides the preload, resulting in a 4-point contact between balls and ball races. This is the most economical method to eliminate backlash but it can reduce efficiency slightly. It may also cause some roughness or torque variation along the screw, especially if screw lengths are more than 50 to 70 times the screw diameter.
Ballnuts with external-tube return often can not achieve smooth operation using a 4-point preload configuration unless spacer balls are added. This reduces load capacity, compared to an equivalent internal-return system. An alternate solution is the use of springpreloaded double nuts. These nuts provide smoothness (due to the 2-point contact) with constant friction torque and assure trouble free operation for ballscrews as small as 3 mm in diameter with 0.5 mm lead. They keep preload constant to the end of service life. However, load capacity, determined by the preload spring, will be low.
Applications for miniature ballscrews fall into four general categories.
Machining. These applications typically use 12 or 16-mm diameter sizes, and in rare cases 8 mm. Both rolled and ground screws are specified. These applications usually involve changing load directions, so backlash must often be eliminated.
Inspection. Inspection equipment, particularly X-Y tables used by the semiconductor industry, require ballscrew drives to provide high accuracy positioning as well as smooth operation during slow scanning speeds, (see the box, “With a focus on smooth positioning, engineers choose miniature ballscrews.”). Poor concentricity of screw and nut can degrade the stability of the table perpendicular to the moving axis. Even if the lead accuracy is not an issue, high accuracy screws are often necessary to reduce side effects such as vibration or stick-slip. Ballscrew sizes usually range from 8 to 12 mm diameter, and the units are mainly precision ground. Ballnuts are preloaded either by ball oversize or with spring-preloaded double nuts for maximum performance with constant velocity at slow scanning speeds.
Micro-manipulation. Where positioning in sub-micron steps, low power consumption, or long service life are the requirements, the ballscrew must reduce friction and eliminate lost motion. If size of the motor drive is a concern, as in micro- surgery manipulators, the ballscrew generally allows very small motors. Although accuracy may not normally be an issue here, it increases efficiency and reduces wear.
Optics. Positioning of optical components often calls for miniature ballscrews. Even if the application doesn’t require absolute accuracy, there is usually a need to avoid any stick-slip effects. Optical instruments, such as laser surface scanning, use high-resolution closed loop controls for high repeatability of even small changes in position. Ballscrews used here are always precision ground, usually with ball oversize preloaded nuts, in diameters ranging from 5 to 12 mm.
With a focus on smooth positioning, engineers choose miniature ballscrews
Ludl Electronic Products Ltd. (LEP), located in Hawthorne N.Y., manufactures high precision X-Y stages for computer controlled inspection equipment, wafer inspection systems, biological applications, quality assurance, material testing, and scanning microscopy. Tables for the X-Y stages are available as open frame designs, with travels up to 25 × 25 in. The tables must provide fast, smooth, and accurate positioning. And they need to be low in profile and lightweight, so they will fit into almost any microscope without alteration. Engineers chose precision miniature ballscrews with spring preloaded double nuts. Nominal screw diameter is 12 mm with leads between 1 and 4 mm for the various application requirements of linear speeds and resolutions.
Through the use of a MAC2000 controller, the tables have a resolution to 0.05 µm, repeatability of better than 1 mm, and traversing speeds from 30 mm/sec for a 4 × 4-in. table to 120 mm/sec for a 12 × 12-in. table.
The miniature ballscrews all have internal ball recirculation and a double nut with integral spring mechanism to keep the balls under a certain preload. This design therefore features 2-point ball contact, which enables smooth and efficient motion, ensures trouble free and accurate operation, and eliminates backlash. The self-adjusting nut keeps friction torque low and constant, enabling the controller to handle low speeds. Engineers program positions by using a manual joystick control. Minimum speeds can be as slow as 2 rpm, resulting in a linear velocity of 0.034 mm/sec. Service downtime is minimal, as adjustments to compensate for ballscrew wear are not necessary.
According to ISO standards, ballscrews are available in five accuracy grades, 1, 3, and 5 for high precision screws, 7 and 10 for normal precision. There are two more accuracy grades, 0 and 2, to meet JIS standard requirements.
George A. Jaffe is executive vice president and general manager, Schneeberger Inc., Bedford, Mass. Alexander F. Beck is president of A. Steinmeyer, Albstadt, Germany.