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Linear actuators based on balls screws efficiently turn a motor’s rotation into precise linear motion with few frictional losses.

One of the most commonly used linear actuators is the ball screw. It can be found in packaging and office equipment, factories for making semiconductors and medical equipment, and a host of other places. Engineers use them based on their load capability, smooth motion, quick accelerations, efficiency, accuracy, and repeatability. Let’s see what gives ball screws these performance advantages.

Ball screw actuator basicsA ball screw linear actuator converts the rotary motion of a motor-driven

screw (torque) into the linear motion of a nut (thrust). In most cases, the screw is held in place, and the nut moves forward and back

BASICS OF DESIGN

Ball Screw Linear Actuators CONTENTS

Ball screw basics … 1

Accuracy … 2

Comparing custom and off-the shelf ball screws … 3

Load capacity… 4

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Sponsored by

NB Corporation of America

on the screw, depending on the direction of screw rotation. The nut’s movements are guided and supported by steel balls traveling in a recirculating track between the nut and the screw. To get useful work out of a ball screw, the nut is usually connected to a block, carriage, or other device that carries a payload. And although ball screws actuators provide movement in just one axis, mounting a ball screw perpendicularly to another atop its carriages lets designers create motion profiles in two or more axes.

THE NB BG BALL SCREW ACTUATOR’S DESIGN FEATURES ENSURE HIGH LOAD CAPACITY AND ACCURACY

PLUS IT’S SO RIGID IT CAN BE ONE-END SUPPORTED.

DamperGrease fitting

Motor bracket

Housing

Guide rail recirculating paths and steel balls

(not visible)

Coupling cover

Ball-screw shaft or lead

Dust cover

Side seal

Side block, which contains the nut

Ball screws have several

components, but they operate more quietly than

hydraulic or pneumatic alternatives and don’t require pumps, hoses, fluids, or shop

air. And unlike pneumatic or hydraulic actuators, ball screws don’t leak, which lowers efficiency and, in the case of

hydraulics, creates a mess, And although belt, cable, or chain-drive alternatives are often less expensive than ball screws, they also are less

precise and stretch as they wear, which makes positioning inaccurate.

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THE NB BG BALL SCREW ACTUATOR’S DESIGN FEATURES ENSURE HIGH LOAD CAPACITY AND ACCURACY

PLUS IT’S SO RIGID IT CAN BE ONE-END SUPPORTED.

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the bottom. So for vertically-mounted ball screw actua-tors or those in working areas where particulates can get in through the bottom surface, designers have to ensure the bellows protect that entry pathway as well.

Contaminants are also unwelcome in the raceways or grooves that carry the recirculating balls. To keep debris and particulates out of the raceways, designers specify wipers, mechanical devices that brush off the balls before they enter the raceways or as they travel through it. The wipers also apply lubricant to keep the balls travelling smoothly. Wip-ers have been shown to extend ball-screw life and enhance machine reliability.

In some cases, engineers use hollow ball screws – ones with a hole down the length of them. The space inside is used to route wires or it can contain coolant. Coolants running through the screw can either help maintain tolerances in precision ball-screw operations or lubricate cutting tools. Hol-low screws also have lower rotational inertia than solid screws, which can dramatically increase starting and stopping speeds.

AccuracyBall screw actuators are generally offered in two or three

levels, grades or levels of accuracy. Commercial grade is the lowest, followed by High or Standard grade, and Precision grade is the highest. To compare various ball screw actua-tors’ accuracy, engineers need to look at each’s positioning repeatability and accuracy, running parallelism, backlash, and starting torque. This is because manufactures have not standardized parameters for the various grades.

Several ball-screw components determine its overall accuracy: the guide rail and raceways, and the screw itself.

A simple calculation determines the torque required to drive a ball screw: T = L P/5.65, where T = torque (lb-in.), L = screw lead (in.), and P = axial load, (lb).

To accommodate the balls, the screw has a helical path spi-raling along its length that forms the inner race for the balls. It matches a similar path, the outer race, machined into the nut. The recirculating balls greatly reduce friction and backlash, and make the ball screw more efficient at moving the nut. Unlike some sliding devices such as power screws that use sliding fric-tion to move a nut along a rotating screw, ball screw actuators use rolling friction which makes ball screws up to 90% efficient at converting the motor’s torque into thrust. It also lets the screw apply and withstand high thrust loads while reducing wear compared to sliding friction. The low level of wear keeps the actuator accurate over its life.

The low-level of friction does mean that ball screw actua-tors can be easily back driven. That is, they can be manually pushed backwards. So brakes must be added in cases where it is imperative for safety or to protect equipment that the load and nut stay in position, or if the ball screw actuator is installed vertically.

Ball screw actuators typically come equipped with metal covers to keep out debris because when the screw is covered with contaminants, it loses accuracy and motion can become jerky. But these covers are not sufficient in work environ-ments where liquids or particulate matter can get into the screw or motor – a common situation in manufacturing and fabrication applications when coolant is used. In such cases, the metal covers are often replaced with accordion-pleated bellows-type covers impervious to fluids and particulate de-bris. They can protect all of the screw and rail guide except

Depending on the application, the speed the load must travel depends on the screw lead of the ball screw and the speed of the driving motor. The shorter the desired travel time, the longer the screw lead must be or the faster the motor speed. However, for the best accuracy, using the shortest lead provides the highest accuracy for the job.

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THE NB BG BALL SCREW ACTUATOR’S DESIGN FEATURES ENSURE HIGH LOAD CAPACITY AND ACCURACY

PLUS IT’S SO RIGID IT CAN BE ONE-END SUPPORTED.

Ball screw basics … 1

Accuracy … 2

Comparing custom

and off-the shelf ball screws …

3Load capacity…

4

fect roundness. And there are no specifications that demands screws be ground for high-levels of accuracy.

Furthermore, to ensure positional accuracy, the balls within the ball grooves of the raceways must not have clearance that lets them deflect. Ball-screw manufacturers usually machine grooves into the screw and nut so that they form one of two shapes that will hold the balls. A slightly elliptical groove lets

The guide rail, the ball-screw’s structural support, and the raceways through which the ball recirculate, must be straight and smooth. So for the most accuracy, guiderails should be precision ground. The same can be said of the slide block (nut). In the past, most manufactures also felt the screws must also be ground, not rolled. However, technological advances in CNC let rolled screws have extremely tight tolerances and near –per-

Comparing custom and off-the shelf ball screw actuatorsREQUIREMENTS CUSTOM DESIGN OFF-THE-SHELF

Pro Con Pro Con

SizeAccommodates

odd sizes

Time consuming and expensive; requires

constant adjustment

Standard size and travel lengths are compact and

economical

Cannot handle odd size requirements

Extreme environments

Can handle vacuum or high temperatures

Covers to protect against fluids and debris are more

expensive

Bellows-type covers are economical and impervious

to fluids and debris

Do not operate in vacuums or high

temperaturesMulti-axis

positioningCan maintain perfect

accuracyExpensive and time

consumingSingle axis actuators

can be stackedStacking can

affect accuracyRepeatable

and absolute positioning

Can handle absolute accuracy for

CMMS or CNC

Units can be 30% larger than off-the-shelf units

Repeatabilities to +/- 1 micron

CMMs or CNC may need absolute

positioning accuracyHigh load / small

footprintCustom nonrecirculating

guide more accurateRecirculating guide can

limit stroke lengthCompact footprint

Large load may deform actuator

Medium to slow load / small

footprint

Lets manufacturers meet unique standards

Expensive and time consuming to find proper

components

Meets most operating standards and is compact

May not meet unique standards

Custom made ball screw actuators

Off-the-shelf ball screw actuatorsCompanies that want to

custom make their own ball screw actuators must first size it to their needs, design, analyze, and test it. Then someone has to purchase the suitable ball screw, carriage, guide rails, supports, and motor brackets. These must be assembled and tested, and an appropriate motor must be acquired and coupled to the screws.

Other companies take advantage of purchasing off-the-shelf ball screw actuators. Manufacturers typically offer ball screw actuators in at least five different sizes with options on travel length, motors, load capability, and other important parameters. These pre-designed or off-the-shelf ball screw actuators are usually more compact than

custom-made versions because the actuator’s guide rail is built in as part of the structure, the slide block includes the nut, and there’s no need for a nut housing. This makes custom-made versions as much as 30% percent larger than off-the shelf ball screw actuators .

Contactarea

CIRCULAR ARCHContact

area

GOTHIC ARCH

LongerGuide rails

Guide rails

Shorter

Load

Load

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rails. The larger and stronger they are (made of steel rather than aluminum, for example), the larger the movable load.

But load capacity also depends on the size of the steel balls recirculating along the grooves between the block and nut or guide rails, as well as the number of balls in contact with the raceways and how they make contact.

Increasing the number of ball circuits is another technique for increasing a ball screw’s load capacity without making the ball screw larger. For example, most standard ball screw actuators have one set of recirculating balls on either side of the block. But doubling the number of ball circuits to two on either side of the block doubles the actuator’s load capacity. The two additional ball circuits would also make the ball screw actuator more rigid than the original two-circuit design.

the balls make contact with the inner surface of the grooves at two opposing points but leaves a small amount of clearance between the balls on their sides that are perpendicular to the contact points. The second shape, called the gothic arch, lets the balls touch the inside of the groove at four points. It also eliminates any clearance between balls that could lead to deflec-tion. This makes the gothic arch best suited for applications requiring maximum precision.

Load capacityOff-the-shelf ball screw actuators have load limits ranging

from 170 to 200,000 lb. The two major factors in determin-ing how much of a load a ball screw can economically move are the size and strength of the screw itself and the guide

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THE NB BG BALL SCREW ACTUATOR’S DESIGN FEATURES ENSURE HIGH LOAD CAPACITY AND ACCURACY

PLUS IT’S SO RIGID IT CAN BE ONE-END SUPPORTED.

Steel balls travel along grooves or thread profiles created when the helical grooves on the screw meet those on the nut. More advanced (and rigid) ball screws use “gothic arches” where the groove has

at least two arcs with a radius somewhat larger than the ball’s, so each ball touches the grooves at four points and there is no clearance between balls. In circular or elliptical grooves, balls touch at two points and there is some clearance between

balls. The number of contact points for each ball determines how the balls move and transmit loads from nut to screw, and vice versa. Therefore, some companies define ball nuts by whether they have two or four contact points. Under this rule, all nuts with four-point contact are “single nut,” and those with two-point contact are “double nuts.” This definition describes nuts from a performance point of view, regardless of design details.

The rigidity and support , and therefore the accuracy, of a ball screw actuator, depends primarily on the composition of the guide rail. It is the actuator’s outer structure and determines how consistently true the raceway’s grooves are. The thickness and strength of the lower edges of the guide rail are critical to its rigidity.

U-shaped outer rails are often specified for more rigidity to resist moment loads. U-shaped rails also limit screw deformation and thereby improve accuracy. Another way to increase the rigidity of the rails is to position them lower than the center of the ball screw, a technique that also gives the actuator a smaller footprint. Lower U-shaped can let designers support the ball screw actuator at only one end.