a recirculating-ball bushing on a shaft and what do you get?
Frictionless movement of the shaft.
But, create grooves along the
axial length of the shaft that correspond to the radius of the bushing's
ball elements and you've got frictionless linear movement coupled with
two important characteristics - those being anti-rotation torque
transfer and higher load capabilities.
Now, that's a ball spline.
It combines a linear bushing (nut) that can handle greater moment loads
and a shaft, which, unlike a rail, can be rotated when needed. It is
useful for high-speed motion and high-speed rotation.
Due to its
variety of application potential, the right ball spline can be found for
a number of automated operations, such as robotics, inspection,
spinning, loading, coating, wire winding, grinding, indexing, die
setting, transferring, conveyance, molding, drafting, measuring, optical
measuring, welding, riveting, printing, book-binding, packaging,
filling and pressing.
Picking the best ball spline for your design
application, however, depends on your knowledge of six key load and
accuracy areas: grooves in the shaft, shape of the grooves, length of
the nut and its raceways, nut tolerances, shaft rigidity and mounting
Key Aspects of Ball Spline Determination
grooves in the shaft. Compared to a slide bush's shaft, a grooved
spline shaft gives greater contact area so load capacity and life is
greater than a like sized slide bush and shaft combination. The basic
dynamic load ratings of a ball spline are typically 5-12 times that of a
slide bush of a similar size.
Click here for larger image.
The number of grooves in a spline
shaft can number from two to six. However, in some instances the six
groove system fills so much of the space on the shaft that there is no
room next to the nut's active ball paths for ball recirculation.
Therefore, the nut has to protrude from the shaft for the balls to
recirculate above and away from the shaft. Also, because the ball
elements will fall out of the nut if the nut and shaft are separated,
this particular type of six groove-shaft ball spline system must be
handled with far greater care.
In the most
popular four groove configuration, the nut can have side-by-side active
and recirculating paths making this a more compact system. Plus, all
ball tracks are in contact with raceways, whereas only half are in
contact in any one direction on some of the six groove shaft systems.
if the load doesn't require six raceways and four will do, space can be
saved.Shape of the grooves.
The 4-point contact design is,
because of its shape, called a gothic arch. The gothic arch eliminates
any clearance that could lead to deflection and is, therefore, best
suited for applications requiring maximum precision.
The 4 point-contact increases the load capacity and rigidity
so that it can handle a greater moment load. Typically, larger spline
sizes utilize the gothic arch 4-point contact grooves.
groove designs on the market, the standard choice is between balls that
make contact with the raceway grooves at two points or at four points
(see Figure 2). A slightly elliptical groove design allows the balls to
make contact at two opposing points but allows a bit of clearance on the
balls' sides that are perpendicular to the contact points. A change of
shaft rotation direction may cause backlash of this circular arc type
nut. Because there are larger contact area differences on a gothic arc,
the inner part of the balls must rotate faster than the outer, which
creates slippage and results in greater friction. For this reason,
circular arc grooves are used for smaller, more friction sensitive ball
Another shaft configuration is available which is
designed for very high loads. It uses semi-triangular protrusions along
the spline shaft. Its main drawback is that it makes it more difficult
for the user to machine the ends of the shaft and it is less stiff. This
type of spline shaft is highly susceptible to vibration since
straightness can be somewhat difficult to maintain. A round grooved
spline shaft is stiffer because of the cross-sectional area. Therefore,
it's better with bending and twisting moments.Length of the nut
and of its raceways.
Since ball raceways are circuits, approximately
half the balls in a raceway are always in the active, load-bearing
portion of the raceway while the other half are in the recirculating
path. The longer the nut, the more active balls supporting the load.
However, some raceways are designed to more efficiently take advantage
of the nuts length and pack more balls into its active portion. The more
active balls in the nut's raceways, the more moment load the nut can
To increase moment load capacity, multiple nuts can be
used in tandem.Nut tolerances.
When raceways are precisely ground
(not drawn) they better conform to the shape of the balls in the nut
and grooved spline shaft. This results in opposing raceways having the
same angles of contact, which eliminates backlash.
between the balls and the grooved spline shaft is controlled by grinding
the nut and grooved spline shaft raceways accurately and also by
installing the proper ball size grade - using the nut's bearing outer
cylinder wall to adjust to the different sizes.
Each spline nut
should be individually preloaded at the factory with larger ball grades
that reduce the clearance between spline shaft groove and nut groove.
Where less vibration and less fluctuation of torque are present,
standard pre-load is sufficient to ensure smooth linear movement. For a
load subjected to minor twisting, alternating moment load and vibration,
a light pre-load is preferable.
Preloading decreases the
available radial play to ensure rigidity, which also increases
precision. This process not only increases the contact area, increasing
direct loading capabilities, it also restricts any radial movement,
increasing the overhung moment capabilities. This creates a sturdier
structure that can handle a very demanding working environment.Shaft
Increasing how symmetrical the spline shaft is, can increase
maximum rotational speed and load capacities as well as reduce
Spline shafts vary as to whether they are
precision-ground, ground or drawn steel bar. They also vary as to the
grade of the base material. Manufacturers rank shafts by characteristics
such as the tolerance of ground shafts, perpendicularity to the end
face, concentricity of the part-mounting section in relation to the
support section, as well as the material grade.
machining of all the shaft grooves so they are linear all the way
through in high accuracies is very difficult. Non-ground (drawn) spline
shafts are, naturally, of lower accuracy.
present three accuracy ratings comparable to High (meaning their
highest precision), Medium (meaning their standard grade - usually a
stock item) and Low (often a non-ground shaft). However, one
manufacturer's top grade can be another's standard grade. Comparing
accuracy grades comes down to comparing measures of the above-mentioned
ranking characteristics - shaft diameter tolerance, straightness,
perpendicularity and concentricity.
If a lesser degree of accuracy
is acceptable because the primary concern is torque transfer, linear
transfer, rotational motion or length, then drawn, non-ground spline
shafts may be the best choice. Some drawn shafts can use the same nuts
as ground spline, but load capacity is reduced because the nut is
traveling in a non-ground raceway groove. However, they're less
expensive and can be as long as five meters, making them appropriate for
material transfer and handling applications
Click here for larger image.
the load is not mounted onto the nut securely, accuracy will be
affected. There are three types of mounting systems.
The standard mode of mounting a cylindrical
nut is with a key. For this system, the cylindrical nut will have a
keyway and separate key. A matching keyway must be bored into the
housing or block that will be mounted on the cylinder nut. It is
critical that the bore precisely fit the key to prevent vibration. The
keyway is important because the nut has to be fixed in a rotational
direction so when the grooved spline shaft is turned the nut isn't
turned within the housing. The housing has to turn with the nut. In
addition to the key slot in the middle of the nut, this system also
requires a snap ring, a push plate or another fixing method to prevent
the nut from slipping out of the housing.
Click here for larger image.
The second mounting system is the flange nut. A
flange nut is much simpler to install because it only requires a rough
bore and mounting holes drilled and tapped to secure the flange to the
housing. Though, to fit the nut into the housing, a hole has to be
bored, this boring does not require the accuracy demanded by a keyed
nut. With the flange type because it is bolted on to the housing, there
is no need for a keyway.
Click here for larger image.
third mounting system is the keyless cylinder nut. It is suitable for
small size compact applications. It is similar to a flange type nut.
Instead of a fixed flange on the nut, there is a squared attachment that
slots into the nut to prevent rotation. The slot can be made with holes
for screws so that the nut will be fixed similarly to the way that the
flange is fixed to housing. But it is much more compact than the
standard flange.Naoki Yamaguchi is the assistant technical
manager, NB Corporation of America