Ball screw

A ball screw ( KGT ) is the totality of a Wälzschraubtriebes with balls as rolling elements. It is used to convert a rotary motion into a longitudinal motion, or vice versa ( as defined in DIN 69051-1 ).

Compared to conventional Gleitgewindetrieben, or so-called Wälzschraubtrieben, there are four advantages:

• Through the point contact of the balls, the driving power is reduced by 2/3

• Less wear of the tracks,
• Increase in the achievable traverse,
• Higher positioning accuracy.

In economic terms are therefore lower maintenance costs, lower processing times and lower scrap rates connected.

• 5.1 adaptronic ball screw
• 5.2 roller screws

Use

The main area of ​​application are machine tools such as lathes, on which the workpiece or tool support must be positioned. The part to be moved is usually secured to the nut and is mounted on linear guides at the same time. Ball screws have almost completely replaced in many areas of mechanical engineering, previously used trapezoidal thread spindles. New designs also open up further application areas that have traditionally found most hydraulic systems are used, such as in presses, injection molding machines and power steering systems.

Design and operation

A motor drives the spindle at either directly or through gear and belt drives. Move between the spindle and nut ( roll off / roll off ) in raceways balls that move axially while rotating the spindle. The return passage in the spindle nut carries the balls back and thus closes the circuit, in which the balls circulate.

Spindle geometry

The circulating balls touch in the ideal case (ie play ) both flanks of the thread at a respective point ( Gothic raceway ). If play is present, can be adjusted.

A characteristic feature of the ball screw is its slope. It is measured in mm and the mean measured in millimeters, straight distance traveled ( stroke) of the spindle nut per revolution of the spindle. This pitch is not the same as the slope of the ramp to that which results from the rolling of the rolling groove on an inclined plane. Those ramp rate is the ratio of the other two sides of the triangle slope ( travel distance per circumference ), a dimensionless number and is usually given in percent. The convention to specify spindle pitch in millimeters and thus normalized to revolution or speed rather than the extent, relieved most calculations for dimensioning of the machine axis, since starting the drive motor usually speeds and torques are transmitted. The slope is comfortable with the speed of the feed in the product. However, the geometry of the rolled-up ramp to the spindle plays a major role in the design optimization of the ball screws.

Ball screws to allow high feed rates under very different loads with high precision. For ball bearings, the disclosure of rolling velocity of the balls has proven to be a criterion for what is technically achievable. The technical limit for ball bearings was in 1996 at around 3000 m / min (180 km / h).

Ballscrews reached this speed at the same time still far, because you can not construct with fixed integrated ball cage the spindle nut. Therefore, " clack " the balls against each other and against the grooves of the nut and spindle. Mainly due to this effect rolls a modern ball screw from typical below 200 m / min. An important limiting factor for the achievable feed so the maximum rolling speed of the ball on the track groove. To as much as possible to take advantage of this potential for stroke, high speed spindles are designed with steep inclines and small diameters. More recent developments include ball screws with ball chain that run relatively quietly, as the above-described effect of the " Klackerns " in these modern systems could be greatly reduced.

The ratio of the slope and extent of spindle characterizing the shape of the helix by mapping to the corresponding unrolled ramp. That ratio is also referred to as a gear ratio of the spindle. According to this definition a ratio of 1:1 means 100 % slope of the ramp. The slope of modern high-speed spindles is a multiple of the respective spindle diameter. Ratios of 3:1 are possible only since 1996. The limitation here results from the need to find a compromise with the requirement of mechanical stability: the spindle transmits with their rolling groove forces. Too thin spindles start swinging under the influence of the radial components of the forces. For the characterization of the distribution of forces pitch angle and load angle are specified according to DIN 69051. For compensation of the disturbing vibrations of thin high-speed spindles specially tuned, spindle nuts inserted in pairs can be constructed that work as a vibration damper.

Dimensional standards for ball screws are only available for the connection dimensions. Therefore, the dimensions of the standard spindles leading manufacturer part vary considerably. Mostly with several different pitches are offered to each spindle diameter available, but in a not too broad framework of the ratio of pitch to diameter.

The standard range on the market ranging from so-called miniature ball screws, the screw pitches are in the range of 1 to 16 mm with spindle diameter 6-20 mm, to screws with gradients of 50 mm and diameters up to 125 mm. Spindle lengths are offered depending on the type from a few centimeters to several meters. Also standardized spindle end processing for the storage of the spindles and the connection to the drive motor are offered frequently by the manufacturers.

Mother geometry

For the geometry of the parent to read the article on the ball nut.

Clearance and Preload

In order to reduce the clearance between the screw and nut to only a few microns or completely eliminate the bias are several ways to choose from. Frequently two threaded nuts are braced against each other.

Other methods for generating the bias are:

• The targeted " filling " of spheres with a defined diameter. This is the nut / spindle pairing either hard or loose braces (note: The balls are never larger than the running groove)

• In the center of the nut, a pitch shift ( pitch-shift ) is incorporated intentionally.

Use

Ball screws can vary depending on shaft diameter and spindle pitch record (kN) up to the three-digit area kN dynamic loads between a few kilo Newton.

The higher rolling friction by bias generates a lot of heat, which affects the precision of the affected machine axis due to the elongation of the spindle at high speeds (eg, movement of a machine in rapid traverse ). High axial forces as well as fast spindle speeds also exert strong pressures on the thread and ball. Under stress, a rubbing of the balls increases together with the necessary torque to move and causes wear. Excessive heating can be hollow with the threaded spindles through which a cooling liquid flows, is counteracted.

Wear reduction

By suitable manufacturing methods and design of the components can be taken of the different types of stress more or less calculation.

Between two normal, bearing balls, a smaller to a few micrometers distance ball can be used in each case. This distance balls exert any pressure on the thread flanks and are therefore not mitzulaufen the constraint but be moved by the load bearing balls against the actual direction of rotation. However, this method of reducing the wear is only a compromise, because the lack of bearing balls is reduced, the axial load capacity of the overall system.

Another way to reduce friction between the balls offer ball cages made ​​of plastic, the realization of which, however, is very expensive, and so are generally only used in high-speed ball screws.

Production

Conventional methods for introducing the grooves in the threaded spindle, threaded vertebrae and grinding wheels. Both the ball and the surface of the raceway surface is mostly hardened, wherein the running groove for high speed spindles is often coated.

Comparison of ball screw and acme screw

• The ball screw can be adjusted against the acme screw with a much smaller game ( " play " ), therefore, has a crucial error smaller lots and are therefore more precise than drive component.
• At the same time the ball screw affects a much smaller friction, although for guides generally represent friction and backlash reciprocal properties. About energy consumption and wear calculation, therefore, result in the use of the ball screw for most constructions relevant savings.
• The second advantage of the low friction of the ball screw is the massively reduced heat generation compared to the trapezoidal threaded spindle during execution of similar tasks. This increases the precision of a suitably equipped machine axis.
• Finally, the low friction of the ball screw provides a smooth with very low break-away force and the resulting very low stick-slip effect. This also directly improves the precision and absolute accuracy of a machine axis.
• The effects permit in sum, that the ball screw drive his machine axis can provide very high feed rates up to 200 m / min at today's state of the art (2007). This reduces production times and production costs and also opens the way to new methods of material processing.

Developments

Adaptronic ball screw

In modern, adaptronic ball screws vary between the nuts built actuator elements and so the bias effect of the heat generation counter and reduce vibrations.

Furthermore dampen ceramic elements in the engine under high dynamic load, the shock and thus contribute to positioning accuracy.

Roller Screws

A refinement of Ball screws are called roller screws, which can be configured as a roller screws with recirculating rollers or a planetary roller screws.

Ball or roller screws turn, are components of so-called actuators, which are widely used in power transmission.