BALL SCREW

Lesson 1: What is a Ball Screw?
— 1 —
Lesson 1: What is a Ball Screw?
We discuss construction of the ball screws in this section.
Section 1 Variety of Screws
Screws
Sliding contact screws
Ball screws
Roller screws
Rolling contact screws
Triangular thread screws
Acme thread lead screws
Others
Triangular thread screws
• Used to fasten two objects.
• Move a nut linearly by rotating a screw.
Acme thread lead screw
• Used to move things or to transfer forces.
• Screw portion of a jack, one of the tools
furnished with a car, is a good example.
Hex bolt Hex bolt
Nut (moving part)
Ball screw Ball nut Would like to operate it more easily!
Development of ball screw

Clipping data
What is a screw?
When you rotate a ball nut around its axis, the ball nut moves in its
axial direction since screw grooves are continuously provided in a
helical form.
Namely, the screw is a mechanical element that converts a
rotational motion into a linear motion. These screws that move
things or transmit forces are the means to convert small rotational
force into large thrust (a force to push).
Lesson 1: What is a Ball Screw?
— 2 —
Section 2 Construction of Ball Screw
1 “Would like to rotate the
screw more lightly and
smoothly!”


By providing steel balls in between the screw shaft and
the nut (grooves), and the balls roll on the grooves (i.e.,
Change to rolling contact from sliding contact to reduce
friction. Refer to the illustration below.).
Ball
Ball nut
Screw
shaft

Note
What is friction force? (Sliding and rolling friction)

When you want to slide a box sitting on a floor, it does not move while your pushing force is yet too
small (static frictional force). But, it starts moving when the pushing force has reached a certain
level. In order to keep the box moving on, you need to maintain your pushing force at its dynamic
frictional force, which is far less than the static friction force.
As described above, the friction force is the force that two objects exert upon each other through
their contact surface and hinder each other's relative movement when they are in contact.

The intensity of frictional force varies with the state of contact. A friction force of rolling contact is
usually smaller than that of sliding contact.
Sliding friction
Acme thread screw
(Requires larger force)
Rolling friction
Ball screw
(Requires far less force)
Lesson 1: What is a Ball Screw?
— 3 —
2 The ball nut moves on the
screw shaft. (Stroke)

It requires a means to
prevent ball from falling off
the ball nut.


Mechanism to recirculate
balls.

Clipping data
Why do the ball screws require ball recirculation parts?
In case of a ball bearing, its steel balls roll only in a circular groove, thus there is no way for steel
balls to go out of it. However, since the groove in the ball screw is helical, its steel balls roll along
the helical groove, and, then, they may go out of the ball nut unless they are arrested at a certain spot.
Thus, it is necessary to change their path after they have reached a certain spot by guiding them, one
after another, back to their “starting point” (formation of a recirculation path). The recirculation
parts play that role.
3 The way the steel balls recirculate endlessly (in the case of return-tube type)
Return tube
Example: 2.5
turns ball
recirculation circuit
When the screw shaft is rotating, as shown in the
illustration, a steel ball at point (A) travels 2.5 turns
of screw groove, rolling along the grooves of the
screw shaft and the ball nut, and eventually
reaches point (B). Then, the ball is forced to
change its pathway at the tip of the tube, passing
back through the tube, until it finally returns to
point (A). Whenever the nut strokes on the screw
shaft, the balls repeat the same recirculation
inside the return tube.
Return tube
Ball nut
Screw shaft
Lesson 1: What is a Ball Screw?
— 4 —
4 Ball screw lead
One
rotation
Travel of ball nut
Screw shaft
Nut
Lead

Lead sizes: The lead is classified into two categories to suit various application.

High helix lead (Large lead) : With this, the ball nut travels a longer distance when the screw
shaft makes one rotation (or the ball nut makes one revolution).
This is suited to high speed operation.

Fine pitch lead : The ball nut travels a shorter distance when the screw shaft
makes one rotation (or the ball nut has made one revolution).
This is suited to highly accurate positioning.
High helix lead
(inter-groove distance is larger.)
Fine pitch lead
(inter-groove distance is narrower.)

Learn the Math!
Relation between lead and rotational speed of screw shaft
[Example]
What is the travel speed of a ball nut with a lead of 10 mm, when its screw shaft rotates at 2000
rpm.?
(10 mm/revolution) × 2000 revolutions/min. = 20000 mm/min. (= 20 m/min)
Lesson 1: What is a Ball Screw?
— 5 —

Coffee Break
The History of the Ball Screw
According to a literature in the 19th century, there was an attempt to replace sliding friction with
rolling friction by means of balls inserted between a male screw and a nut, namely this is a concept
of the ball screw, in order to rotate a screw of driving mechanism more lightly (illustrated below).
Because of technological level of those days, however, they could not practically apply the idea.
The Saginaw Division of General Motors in the United States used ball screws practically for the
first time in automobile steering gears in the 1940’s. Since then design and production technology
for ball screws have made great advancement.
In Japan, as mechanical industries advanced, the call for ball screws grew louder. Responding to
these voices, NSK took the initiative to develop ball screws using its expertise in ball-bearing
design and manufacturing, and the company eventually succeeded in launching the first ball screw
type automobile steering gears in Japan in 1958.
Although the main application target for the precision ball screws was NC machine tools, the first
job for which it was used was to convert acme thread lead screws of the X, Y and Z axes of a
milling machine called K5, manufactured by Makino Milling Machine Co. Ltd., into ball screws.
This K5 model was the best-selling brand in the industry back then, and over fifty machines were
produced monthly. The NSK precision ball screws were used for the first time in them.
Thereafter, due to ever-progressing improvement in design techniques and manufacturing
technologies, as well as needs for streamlining production in general, the high performance
characteristics of ball screws soon made them one of the vital elements of NC machines,
laborsaving machinery, and so on.
Introduction of ball screw in The Practical Engineer, December 1898
(R. K. Allan, Rolling Bearings)
Lesson 2: Characteristics of Ball Screws and Application Examples
— 6 —
Lesson 2: Characteristics of Ball Screws and
Application Examples
Since ball screws feature application of rolling friction, they have various advantages (features) compared
with sliding contact screws. Given below are explanations, with emphasis on the application.
Ball screws are used where motion
direction must be changed
(converted).

From rotations to linear motion

From linear motion to rotations
1 High mechanical efficiency
Most (90% or more) of the force used to
rotate the screw shaft can be converted
to the force to move the ball nut.
(Since friction loss is extremely low, the
amount of force used to rotate the screw
shaft is as low as one third of that
needed for the acme thread lead screw.)


A piston connected with the screw shaft moves while the ball nut is driven by a servo motor.
This illustration shows a case in which the ball nut rotates and the screw shaft moves.
There is also another case of this application in which the screw shaft rotates and the ball nut
moves.