**Back Drivability:** With reversible driving system a force applied to the stage carriage can generate motion, when its motor is not powered. Such stages are called back drivable. Stages with direct drives are back-drivable with a low force: Stages with ball-screw are back-drivable, if the applied force is above a given threshold depending on the ball screw characteristics.

Stages with lead-screw are generally not back-drivable, unless pitch of the screw is very high. If the stage is back drivable and used in vertical applications, a payload-dependent force is constantly applied, resulting in downward motion of the stage carriage at power off. Unless otherwise specified, the back-drivability threshold of Newport stages is above the Axial Load Capacity.

**Off-Center Load Capacity:** The maximum load capacity of a stage is diminished when the load is not centered (refer to the stage specifications section in the catalog for off centered load equations and/or contact Newport’s Applications Engineers to discuss your specific load conditions).

**Maximum Inertia:** Inertia is the measure of load’s resistance to change in velocity. The larger the inertia, the greater the force required to accelerate or decelerate the load. If there is a constraint on the amount of force available, then the allowable acceleration and deceleration must be adjusted to an acceptable value. Inertia is a product of mass elements and the square of their distance from the axis of rotation. The maximum inertia specified for rotary stage is a value based on available torque (limitation in acceleration) and bearing stiffness (limitation in natural frequency and associated vibrations).

**Speed (Velocity):** Speed (Velocity) is the rate of change of position. The maximum speed specification is provided at the stage’s normal load capacity. Higher speeds are possible for lower loads or larger motor drivers. Minimum speeds are highly dependent on a motion system's speed stability.

**Speed Stability:** Speed stability is a measure of the ability of a motion system to maintain a constant speed within specified limits. It is usually specified as a percentage of the desired speed. Also specified as velocity regulation, this parameter depends upon the stage’s mechanical design, its feedback mechanism, the motion controller, control algorithm, the magnitude of the speed, and the application.

The actual speed of a moving part is usually not measured directly. Instead, it is calculated based on a sample of the position. As a result, the value of speed stability depends a lot on the sampling frequency. In order to be accurately defined, speed stability should be specified in a given bandwidth.

**Acceleration:** Acceleration is the rate of change of velocity. Unless otherwise specified, Newport sets the acceleration of its stages to a value that enables the stage to reach the maximum velocity in 250 ms, so maximum acceleration = maximum speed * 4/s.

**Jerk:** Jerk is the rate of change of acceleration.

**Mean Time before Failure:** Mean Time before Failure (MTBF) is the prediction of product reliability. Tests and statistical analysis of parts and components are performed to predict the rate at which a product will fail. It is one of the most common forms of reliability prediction and is usually based on an established analytical model. Many models exist, and choosing one over another must be based on a broad array of factors specific to a product and its application. In general, MTBF is specified with a duty cycle parameter.

In the case of simple system with a constant failure rate (number of failures/unit time), the fraction of units still working after a time equal to the MTBF is 37%.