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Cover StoryJONATHAN KASBERG |Program ManagerNook Industries, Cleveland, Ohio

The Fundamentals of

BALL SCREWSProper ball-screw selection involves a numberof design and application considerations.

tancy of ball screws. However, contamination, lubrication,

and improper mounting and installation techniques can also

shorten a ball screws lifetime. For manufacturers looking to

extend the life of their screws, its often beneficial to order a

larger-size screw to handle a larger load.

Life expectancy, L, can be calculated from the following

equations:

where L= life in revolutions; Ca= basic dynamic-load rating;

Fm

= equivalent axial load (N); andfw= fatigue factor.

Calculate the equivalent axial load, Fm, using:

where Fi= each increment of axial load (N); q

i= percent of

stroke at load Fi; and n

i= rotational speed at load F

i(rpm).

The final variable nm

= equivalent rotational speed (rpm)

and is calculated using:

As an alternative to hand calculations, charts can help deter-

mine life expectancy:

For metric ball screws: http://www.nookindustries.com/

LinearLibraryItem/Ballscrew_Life_Exp.

For inch versions: http://www.nookindustries.com/LinearLi-

braryItem/Life_Expectancy__SRT_Rolled_Inch_Screws.

Ball screws handle both compression and tensile loads.

Compression loads tend to compress or squeeze the screw

axially, which can make s crews bow out. Tensile loads, on the

other hand, often stretch the screw axially. While compres-

sion loads can be more problematic, tensile loads may cause a

screw to elongate and crack.

In addition, its important to note that screws are only

designed for thrust loads or straight-line axial thrust motion.

Any type of overturning or side load could reduce screw life

by up to 90%.

MOUNTING CONSIDERATIONS

Determining the screws load direction as well as the type

of mounting (end fixity) helps when selecting s crews, as engi-

neers must account for both critical speed and buckling (dis-

cussed later). The end fixity of bearings defines how a screw is

supported at both ends. The degree of end fixity relates to the

amount of restraint at the ends of a screw. End fixity breaks

down into three basic types: free (no support); simple (sup-

ported at a single point); and fixed (rigidly restrained). End

bearings give the screw stability and rigidity. Fixity also affects

critical speed, column loading, and overall performance.Multiple or spaced pairs of bearings are more rigid than

simple supports. However, they arent truly fixed because of

their inherent compliance. A screw can be supported with dif-

ferent end-fixity combinations, such as:

One end supported with a double bearing, the other end

free. This setup is not recommended, except for applications

with short travels and slow speeds.

One end with a double bearing, the other with a single

bearing.

Both ends with double bearings.

Both ends supported with quad bearings. Note that in such

cases, the screw is highly rigid and extra care should be

taken to ensure compliance.

When using fixed bearing mounts on both ends, experts

recommend reviewing the setup with the ball-screw manu-

facturers application engineers to determine the mount-to-

mount length tolerance of the final assembly.

CRITICAL SPEED

The speed that excites the natural frequency of the screw

is referred to as the critical speed. The screw will resonate

regardless of screw orientation or whether the nut rotates

about the s crew.

Critical speed varies with diameter, unsupported length,

end fixity, and speed (rpm). Shaft straightness and assembly

alignment can also play a role; therefore, manufacturers rec-

Of all the screws used for industrial motion

control, ball screws rise to the head of the

class, offering several advantages over oth-

er options such as roller screws and acme

screws. Featuring efficiencies of at least 90%, ball screws are

one of the most economical ways of converting rotary motion

into precision linear motion. Theyre able to move heavy

loads at fast speeds with outstanding accuracy. Moreover, ball

screws can be cost-effective alternatives to pneumatic andhydraulic actuators.

A ball screw consists of a threaded shaft and a nut, and either

one can act as the traversing component. Ball screws work in

a similar fashion to ball bearings, where hardened steel balls

move along an inclined-hardened inner and outer race.

To select the correct ball screw and nut for a specific appli-

cation, engineers should consider design factors like loads, life

expectancy, speed, length, and mountings. Application con-

siderations include lubrication and environment.

LOADS AND LIFE

Several loading considerations impact ba ll-nut screws. For

instance, static loads are generally straightforward and reach

the maximum limit when screw and nut are loaded to their

capacity without damage.

Dynamic loads are thrust loads that, when applied to theball nut and rotating screw assembly, result in a specific mini-

mum life. Dynamic-load ratings for metric screws indicate a

load in newtons that can be carried for 1 million revolutions.

Inch/imperial screws, which factor in the lead of the screw,

have dynamic-load ratings for 1 million inches of linear travel.

Due to their steel-on-steel design, bearing manufactur-

ers have developed techniques to calculate the life expec-

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Ball screws are noted for efficiently

moving high loads with outstanding

accuracy. Here, theyre used in so lar

arrays that track the sun.

Three widely used types of ball nuts are (left to right) cylindrical body with key, flanged, and v-thread.

The flanged configuration is common to all ball nuts, and actuator companies and OEMs often use the

cylindrical body with key style. V-threads are custom-made to be threaded into or attached to a flange.

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Backdriving:When loading ball screws, its important to

remember backdriving. Backdriving occurs when a motor-

driven ball screw free-falls once the motor shuts off. To avoid

this, install brakes on motors or use safety pins to lessen the

risk and catch the load. Acme screws, which have lower effi-

ciency, are less likely to experience backdriving.

Environment: Because ball screws often work in environ-

ments exposed to dirt and debris, manufacturers should take

precautions to keep out contaminants and prevent premature

failure. Screws often are coated with a thin dense chrome,

black oxide, or nickel-plated finish as a first line of defense.

Another option is to equip ball screws with bellows boots

that cover the screw and exclude contamination. As the nut

traverses, the boot expands and contracts like an accordion.

Bellows boots come in various materials that suit everything

from light-duty to the most extreme applications.

Wipers offer another form of protection. Nut wipers made

of felt or plastic brush the nut free of dirt and other debris and

keep contaminants from entering the ball nut.

Lubrication: Ball-screw applications vary widely, so theres

no definitive recommendation for the type and amount of lubri-

cation needed. However, factors such as frequency of use, tem-

perature, and viscosity are essential considerations for lubrica-

tion options. While a light oil or grease suits most applications,

avoid using lubricants containing molydisulfide or graphite.

A good rule of thumb is to always apply enough lubrication to

maintain a thin lubricant film between the nut and screw.

ommend that maximum speed be limited to 80% of theoreti-

cal critical speed.

Calculate critical speed, ncr, in rpm from:

where L= the distance between supports (mm); E= the

modulus of longitudinal elasticity (2.05 105N/mm2; I=

minimum second area moment of inertia of screw shaft

cross section (mm4), where I= (dr4)/64; d

r= screw shaft

root diameter (mm);g= acceleration of gravity (9.81 103

mm/sec2); = specific weight (7.71 10 -5N/mm3); andA=

minimum cross-sectional area of the screw shaft (mm2),A

= dr

2/4.

In addition, = a factor determined by the ball-screw sup-

port method, where:

One end fixed and the other free, = 0.59.

Both ends simply supported, = .

One end fixed and the other simply supported, = 1.25.

Both ends fixed, = 1.49.

Charts to help calculate critical speed can be found at:

Metric: www.nookindustries.com/LinearLibraryItem/

Ballscrew_Life_Exp

Inch: www.nookindustries.com/LinearLibraryItem/Criti-cal_Speed__Ball_Inch_Screws

COLUMN LOADING

When a screw loaded in compression exceeds its limit of

elastic stability, the screw will fail through bending or buck-

ling. To calculate the column strength to verify a screw can

carry the required load without buckling, solve for:

Fc= (C 2 E I)/L2

where Fc= permissible axial load to buckling (N); and C= a

factor determined by the ball-screw support methods, where:

One end fixed and the other free, C= 0.25.

Both ends si mply supported,C= 1.

One end fixed and the other simply supported, C= 2.

Both ends fixed, C= 4.

If the selected screw does not meet compression-load crite-

ria, consider the following options: Change end fixity; for example, from simple to fixed.

Design to use the screw in tension.

Increase screw diameter.

Charts to help calculate column loading can be found at:

Metric: www.nookindust ries.com/LinearLibraryItem/Col_

Stren_w_EZM

Inch: www.nookindustries.com/LinearLibraryItem/Column_

Strength__SRT__XPR_and_SGT_Inch_Screws

APPLICATION CONSIDERATIONS

Accuracy: Because manufacturers use different processes to

make ball screws, engineers have several options for weighing

accuracy versus cost.

Rolled screws are manufactured in a process that uses rotat-

ing dies to deform round metal bars and generate a helical

thread pattern. This is a cost-effective manufacturing method,

but typically produces screws with lower accuracy, compared

to ground screws. Some manufacturers do, however, produce

high-accuracy screws via tightly controlled rolling processes.

Ground screws are made from a process by which a grinding

wheel cuts screw threads into case-hardened material. As a

result, lead accuracies are typically much tighter than in rolled

screws. Ground screws are generally preferred over rolled ver-

sions in aerospace applications, because the seam formed close

to the major diameter of the screw shaft is thought to have

potential for crack propagation.

NOTES ON PRELOADING

PRELOADING IS THE result of an internal force introduced

between a ball nut and screw assembly that eliminates free

axial and radial lash. There are three methods for preloading:

The double-nut method uses two ball nuts loaded in

opposing directions by a spacer, so that they dont wiggle

when stationary.

Lead shifting manufactures a shift or offset in the lead of

the ball nut. For example, a lead might be shifted from 5.00

mm to 5.05 mm to shift the ball bearings in a different direc-

tion inside the ball nut. This is the preferred method when

considering compactness, but it reduces load capacity.

Ball selection is a low-cost method that involves using pre-

selected, oversized ball bearings to create four points of con-

tact between the nut and screw. This permits heavier loads,

but friction from the contact can reduce bearing life.

BALL-SCREW BASICS

FOR ANY ENGINEER working with ball screws, it helps to

understand some key terminology.

Ball-circle diameteris the diameter of the circle created

by the center of the ball bearings when they contact both the

screw and nut.

Root diameteris the minimum diameter of the screw

measured at the bottommost point of the threads. Both

ball-circle and root diameters are important when calculating

application characteristics and sizing parameters for factors

such as column loading and critical speed.

Pitchis the axial distance between two consecutive

threads on a screw.

Leadis the linear distance traveled by the nut or screw

during one full rotation.

Startsare the number of independent threads on the

screw shaft. Screws typically have one, two, or four starts,

which resemble a helix that wraps around the shaft. Th e

pitch multiplied by the number of starts equals t he leadof

the screw.

Lashis the result of axial movement between a nut and

screw without rotation. While lash can disrupt the accuracy of

the screw, it typically occurs without any serious issues. Nor-

mal screws come with a relative amount of lash, and screws,

which are only loaded in one direction, wont be affected by

lash. Lash can be controlled through preloading.

Mechanical

Mechanical

Critical speed, the speed at which a screw resonates, varies with

unsupported length, diameter, end fixity, and rotational speed.

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