Ballscrew
Design Consideration
STATIC LOAD
Static load rating is the maximum non-operating load capacity. In
all applications the total weight of the load should not exceed the
static load capacity of the ballscrew. If a ballscrew is subject to
a tension load, the static load capacity of the screw should be verified.
However, if the screw is subject to a compression load, one that tends
to buckle the screw, both the static and compression load capacities
of the screw must be verified.
APPLIED LOAD
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The applied load (the load seen by the screw) is dependent on whether
the screw is to be used in a horizontal or vertical application, the
total weight of the load that has to be moved, and the number of ballscrews
used to move the load.
Whenever possible, the load direction should be coaxial with the
screw. Overturning or cocking type loads should be avoided. In vertical
applications, such as lifting or jacking, the applied load is equal
to the total weight of the load that has to be moved. In horizontal
applications the load seen by the screw is equal to the total weight
of of the load times the coefficient of friction for the type of ways
used to guide the load as it moves.
Most applications only require one ballscrew. However, if more than
one screw is required, the load seen by each screw is dependent on
the distribution of the weight between the screws. For example, if
the weight is equally distributed among four screws, the load seen
by each screw would be one-fourth the total load.
DESIGN LIFE OBJECTIVE
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After determining the applied operating load, the next step is to
determine the design life objective, measured in inches of travel,
for the application. For a horizontal application, the formula for
calculating the design life objective is:
Design Life = S x C x H x D x Y
Where:
S = Stroke Length, in inches C = Cycles per hour H = Operating hours
per day D = Working days per year Y = Expected design life for the
application, in years
For a vertical application, the above formula must be multiplied
by 2 because the load is always applied to the same side of the ball
groove.
LOAD / LIFE RELATIONSHIP
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The load-life relationship for a particular size ballscrew is an
inverse cube ratio, analogous to the B10 rating common to the ball
bearing industry. For example, reducing the load on the ballscrew
by 1/2 increases the life expectancy 8 times. Doubling the load decreases
the life to 1/8 the original expectation.
This makes the life of a ballscrew predictable based on the applied
operating load that has to be moved and the total inches of travel
defined by the design life objective.
In most ballscrew applications, where the load is in tension, selection
of the proper size ballscrew can be made based on the load-life relationship
for a stated load and design life objective. However, in those applications
were column loading, critical speed, or other factors come into play,
additional considerations must be made.
CRITICAL SPEED
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Critical speed is the speed at which the nut or screw has a tendency
to develop severe vibrations. Under normal operating conditions, the
maximum safe operating speed of a ballscrew assembly is 80 percent
of the critical speed rating for the screw.
Critical speed is a function of the unsupported length of the screw
diameter, and the type of end bearing supports. The unsupported length
is the dominate factor, because the critical speed is inversely proportionate
to the square of the unsupported length.
COMPRESSION LOADING
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Compression or column loads have a tendency to cause a ballscrew
to buckle. Therefore, whenever a ballscrew is subject to these types
of loads, the safe compression rating of the ballscrew must be verified.
The compression load capacity is dependent on the unsupported length
of the ballscrew, the type of end bearing fixity, and the ballscrew
diameter.
