An Explanation of the Basics
LEAD VS PITCH
Pitch is the axial distance between threads. Pitch is equal to lead in a single start screw. There may be more than one thread “strand” on a single screw. These are called starts. Multiple start lead screws are usually more stable and efficient at power transmission. on of the screw. Throughout this catalog, lead will be the term used for specifying a screw as it is the linear distance travelled for one revolution of the screw. The larger the lead, the more linear distance travelled per one revolution of the screw.
LOAD
Typically quantified as either lbs OR kg to move or pounds force (lbsF) or kgF for thrust.
VELOCITY (V)
Typically quantified as either inches/second (mm/sec) required for your application.
DISTANCE
Typically quantified as either inches or mm, is the required move distance.
TIME (T)
Typically quantified in seconds. Time period required for a given distance defines the velocity, acceleration (A), and deceleration needed to reach commanded position.
HORIZONTAL OR VERTICAL APPLICATION
Vertical orientation applications add the potential problem of backdriving when power to the motor is off and without an installed brake. Vertical applications also have an additional gravity factor that must be part of the load/force calculation.
ACCURACY OF SCREW
Specified as a measurement over a given length of the screw. For example: 0.0006 in per inch. Lead accuracy is the difference between the actual distance travelled versus the theoretical distance travelled based on the lead. For example: A screw with a 0.5 inch lead and 0.004 inch per foot lead accuracy rotated 24 times theoretically moves the nut 12 inches. However, with a lead accuracy of 0.004 inch per foot, actual travel could be from 11.996 to 12.004 inches.
TOTAL INDICATED RUNOUT
The amount of “wobble” around the centerline of the screw.
REPEATABILITY
Most motion applications put the most significance on the repeatability (vs accuracy of screw) of a system to reach the same commanded position over and over again. For example: A repeatability of ± 0.005 inch means that after repeated commands to reach the same target position, the linear error will be no more than ± 0.005 inch.
TENSION OR COMPRESSION LOADING
A load that tends to stretch the screw is called a tension load. A load that tends to “squeeze” or compress the screw is called a compression load. Depending on the size of the load, designing the screw in tension utilizes the axial strength of the screw versus column loading.
RADIAL LOAD
A load perpendicular to the screw. This is not recommended unless additional mechanical support such as a linear guide is used.
AXIAL LOAD
A load that exerted at the center line of the lead screw.
STATIC LOAD VS DYNAMIC LOAD
Static load is the maximum thrust load, including shock load, that should be applied to a non-moving screw.
Dynamic load is the maximum recommended thrust load which should be applied to the screw while in motion.
Click for more on static load vs dynamic load.
BACKDRIVING
Backdriving is the result of the load pushing axially on the screw or nut to create rotary motion. Generally, a nut with an efficiency greater than 50% will have a tendency to backdrive. Selecting a lead screw with an efficiency below 35% may prevent backdriving. The smaller the lead, the less chance for backdriving. Click for more on backdriving.
TORQUE
The required motor torque to drive just the lead screw assembly is the total of:
Inertial Torque
Drag Torque (friction of the nut and screw in motion)
Torque to move load
LUBRICATION
The nut material contains a self-lubricating material that eliminates the need for adding a lubricant to the system. The Teflon coated screw option also lowers friction and extends life of the system.
END MACHINING OF THE SCREW
Standard metric or English options are available. Custom end machining specifications are also available on request.
FIXITY
The performance (speed and efficiency) of the screw system is affected by how the screw ends are attached and supported.
COLUMN STRENGTH
When a screw is loaded in compression, its limit of elastic stability can be exceeded and the screw will fail due to bending or buckling.
CRITICAL SPEED
Critical speed is the rotational speed of the screw at which the first harmonic of resonance is reached due to deflection of the screw. A system will vibrate and become unstable at these speeds. Several variables affect how quickly the system will reach critical speed:
The lead of the screw
The rotational speed
End fixity
Thrust load
Diameter of the screw
Tension or compression loading
For example, the following chart shows that for a screw with a 3/4 inch diameter and 70 inch length, the threshold for critical speed is 700 RPM.
BACKLASH
Backlash is the relative axial movement between a screw and nut at standstill. It is normal for backlash to increase with wear over time. Backlash compensation or correction can be accomplished through the application of an anti-backlash nut. Backlash is usually only a concern with bi-directional positioning.
Anti-backlash nuts available upon request.
LINEAR MOTION SYSTEM TYPES
A. Non-Captive
B. External Linear
C. Captive
