Mechanical Definition :

Accuracy:
The degree of conformity of a measured/calculated quantity to its actual (true) value.

Axial Play or End Play:
The maximum relative displacement of the bearing rings, in a direction parallel to the axis of raotation.

Back Drive:
The result of a load pushing axially on the screw or nut to create rotary motion. A screw with greater than 50% efficiency will most times have a tendency to back drive. With regard to lead screws- if the lead is less than 1/3 the diameter the screw will not back drive.

Backlash:
Free play in an axial assembly along the path of travel. Specifically, the rotational shaft movement that results in no linear movement of the linear way.

Flatness:
Variation from the zero point perpendicular to the axis of travel

Lead:
The axial distance traveled by the nut in one revolution of the screw. The lead can be calculated by multiplying the pitch times the number of starts.

Repeatability ( Also called Precision)
The degree to which further measurements or calculations will show the same or similar results.

Starts:
The number of helical thread elements on a single screw shaft.  Multiple starts are used to increase the lead.
Motor Definition
Load Definition :

Rated Dynamic Load (C):

This term is arrived at based upon evaluation of a number of identical linear systems individually run in the same conditions. 90% of them can run with the load (with a constant value in the constant direction) for a distance of 2,000,000 inches without damage caused by rolling fatigue.

Rated Static Load (C0): 

This term defines a static load such that at the contracting position where the maximum stress is exercised, the sum of the permanent deformation of the rolling elements and that of the rolling plane is 0.0001 times the diameter of the rolling element.
Coeffiecient of Friction
Linear System Type
Models
Coefficiant of friction
Recirculating Bushings
SW, SWK, SWF
0.002 to 0.003
Linear Motion Bushing
TW
0.002 to 0.003
View Vertical Load, Side Mounted Load, Horizontal Load
Holding Torque
The maximum restoring torque that is developed by an energized stepper motor if the shaft is rotated by external means. The windings of the motor are on, but they are not being switched.

Microstepping:
When motor steps are electronically divided into smaller steps. Motors are typically divided into 200 steps, each step being 1.8 degrees of a full revolution. This angle is known as the step angle.

Step Angle:
The nominal angle through which the step motor shaft rotates between adjacent steps.

Stepper Motor:
A stepper motor is a special type of electric motor that moves in increments, or steps, rather than turning smoothly as a conventional motor does. The size of the increment is measured in degrees and can vary depending on the application.

Servo Motor
A direct current motor that is operated in a closed loop system (servo circuit). Vibration less, smooth running, broad speed range and very good low speed torque are characteristics of DC servo motors.
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