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Pivot Bearing Terminology
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Axial Load (Pa)
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A pure tension or compression load acting along the longitudinal axis of the pivot and is also referred
to as "Thrust Load".
SHOW SKETCH
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Axial Rate
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The number of pounds of axial (thrust) load necessary to create a specific unit of relative motion
between the flexure-connected portions of the pivot in the longitudinal direction. It is usually stated
as pounds per inch.
EXAMPLE - an axial load of 50 pounds is found to create a deflection of 0.001 inches; the axial rate is
50 pounds divided by 0.001 inches or 50,000 pounds per inch.
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Center Shift
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The difference between the diametrical centers of the rotated portion of the pivot and the portion
(or portions) that are fixed as the pivot is rotated. Center shift is a kinematical characteristic
that occurs as a function of the angle deflected for examples on how to calculate center shift
refer to our Flexural Pivot Engineering Data Handbook. An auxiliary center shift is caused by
radial loads (see "Radial Rate").
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Compression (Vc)
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A radial load directed to the pivot such that the loads are transferred through the flexures by
compression. Excessive compression loading can cause the flexures to buckle.
SHOW SKETCH
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Cycle
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One cycle is defined as the motion from the neutral position to the positive displacement,
back through the neutral position to the negative displacement, and returned to the neutral position.
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Horizontal Load (Vh)
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A radial load that bisects the crossed flexures and is parallel to the travel slots.
SHOW SKETCH
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Hysteresis
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Is the total amount of variation from neutral position when a torque is applied such as to produce a
complete cycle. It is a property of systems that do not instantly react to the forces applied to them,
but react slowly, or do not return completely to their original state. It is expressed in minutes or
seconds, as applicable. The amount of "non-return" to its starting position.
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Linearity
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The degree of which a plot of torque vs. deflection is a straight line.
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Moment
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The torque applied in a given plane through a pivot other than around the longitudinal axis.
It is usually considered as a torque caused by an offset or overhung Vc, Vt or Vh load.
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Non-Linearity
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The degree of which a plot of torque vs. deflection varies from a straight line.
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Neutral Position
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The position of zero angular travel when the pivot is under no load, with no applied torque about any axis.
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Neutral Shift
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The change in the neutral position due to effects other than applied loads and/or torques, such as temperature.
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Plane Load (Pc, Pt)
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A radial load directed exactly in line with a pivot flexure. Pc is a radial load that results in a
compressive stress in the flexure and Pt is a radial load that results in tensile stress in the flexure.
SHOW SKETCH
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Radial Load (Vc, Vh, or Vt)
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A load directed to the pivot center from any angle around the pivot circumference.
SHOW SKETCH
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Radial Rate
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The number of pounds of radial load necessary to deflect the pivot structures
(as measured at the outer diameter) a specific unit of measure. It is usually stated as pounds per inch.
EXAMPLE - a radial load of 10 pounds is found to deflect the pivot 0.0001 inches; the radial rate is
10 divided by 0.0001 inch or 100,000 pounds per inch.
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Radian
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A unit of angular measure, there are 2Π(Pi) radians in every 360° (circle),
thus one radian is approximately 57.3 degrees.
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Spring Rate
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The amount of force needed to deflect a spring through a unit of travel,
usually expressed in pound-inch per degree.
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Tension Load (Vt)
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A radial load directed to the pivot such that the loads are transferred through the flexures
by tension. Excessive tension loading can cause the brazed joints to fail.
SHOW SKETCH
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Thrust Load
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See "Axial Load".
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Torsional Spring Rate (K)
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The amount of torque necessary to rotate the pivot per unit of rotating motion in terms of
pounds-inches per degree.
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Torsional Rate Equation
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T=K*θ
In which (T) equals torque, (K) equals torsional rate, and (θ) equals angular deflection.
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Translate
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To move an object sideways in a direct straight line without rotation.
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