Terminology

Lead

Lead is the axial distance the nut advances over one revolution of the screw. Throughout this catalog, lead will be the term used for the linear distance traveled over one revolution of the screw.

The larger the lead, the greater the linear distance travelled per one revolution of the screw. Lead = Pitch x number of screw starts.

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.

Lead screws with multiple starts are usually more stable and efficient at power transmission.

Accuracy of Screw

Specified as a measurement over a given length of the screw.

For example: 0.004 inch per foot.

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 accruracy of 0.004 inch per foot, actul travel could be from 11.996 to 12.004 inches.

Position Tolerance
The error between actual distance travelled vs theoretical distance travelled.
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.
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 application also have an additional gravity factor that must be part of the load/force calculation.

Total Indicated Runout
The amount of “wobble”around the centerline of the screw.
Vibration and Noise

The hybrid stepper motor’s resonance will be occured when pulse is up to 200PPS.

Try starting your acceleration ramp at above these levels. Micro-stepping will also help through these ranges.

Static Load
Load applied to the screw when the screw is standstill.
Driver

Stepper motors require some external electrical components in order to run. These components typically include a power supply, logic sequencer, switching components and a clock pulse source to determine the step rate.

Many commercially available drives have integrated these components into a complete package.

Very basic stepper drivers have various functions and it can generate high performances. DINGS already has wide range of driving part so please consider to take and use.

Dynamic Load
Load applied to the screw when the screw is moving.
Holding Torque
When motor is static and rated current is applied to two phase, the stator’s holding ability to the rotor.
Rotor Inerita
Moment matters when accelerating or decelerating
Heat Rising
Rise in temperature of motor body while in use after a certain period of time.
Response Per Step
Time taken to complete one step.
Step
Characteristics of stepper motor that the rotor moves step by step as the stator commutates phase by phase.
Step Angle
Angular movement of every step.
Pull Out Torque
Varying with the drive condition (frequency and current), the maximum load the motor can handle without missing a step.
Pull In Torque
The maximum torque (including friction) under which a motor will accelerate from rest to constant speed.
Efficiency
The ability of a mechanial system to translate an input to an equal output.
Resolution
Incremental linear distance the actuator’s (motor) output shaft will move per input pulse.
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.

compression loading image tension loading image
Radial load

A load perpendicular to the screw.

This is not recommended unless additional mechanical support such as a linear guide is used.

radial load image
Axial load

A load that exerted at the center line of the lead screw.

axial load image
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 effciency below 35% may prevent backdriving. The smaller the lead, the less chance for backdriving or free wheeling.

Vertical applications are more prone to backdriving due to gravity.

Torque

The required motor torque to drive just the lead screw assembly is the total of:

  • 1. Inertial Torque
  • 2. Drag Torque (friction of the nut and screw in motion)
  • 3. Torque to move load
Lubrication
The nut material (Delrin) contains a self-lubricating material that reduces 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 upon request. Please contact your local DINGS' representative.

Fixity

The performance (speed and efficiency) of the screw system is affected by how the screw ends are attached and supported.

Type of End Fixity Relative Rigidity Critical Speed Factor Critical Load Factor
less rigid image Less Rigid 0.32 0.25
rigid image Rigid 1.0 1.0
more rigid image More Rigid 1.55 2.0
most rigid image Most Rigid 2.24 4.0
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:

  • 1. The lead of the screw
  • 2. The rotational speed
  • 3. End fixity
  • 4. Thrust load
  • 5. Diameter of the screw
  • 6. 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 700RPM.

Critical rotation speed (rpm) vs. Unsupported screw length for various screw diameters (inch)
Critical rotation speed (rpm) vs. Unsupported screw length for various screw diameters (inch) graph
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 or an anti-backlash nut.

Backlash is usually only a concern with bi-directional positioning.

backlash image