Technology Overview
One of the most common methods of moving a load from point A to point B is through
linear translation of a motor by a mechanical lead screw and nut. This section is here to assist and refresh your
understanding of the basic principles of lead screw technology prior to selecting the system that is best for your
application.
Some basic design consideration are as follows:
1. What is the load of your system?
2. What is the required speed to move from point A to point B?
3. What is the distance to be travelled?
4. What accuracy does your application require?
5. What is the required time to move from point A to point B?
6. What repeatability does your application require?
7. Horizontal vs vertical orientation?
TERMINOLOGY
LEAD
Lead is the axial distance the nut advances on one revolution
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 revolutions of the screw. The larger the lead, the more
linear distance travelled per one revolution of the screw. Lead = Pitch x screw start.
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.
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 approach value 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.
TENSION OR COMPRESSION LOAD
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 colomn
loading.
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 still.
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. Some basic drive units have
only the final power stage without the controller electronics to generate the proper step sequencing.
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 INERTIA
Moment matter when accelerate or decelerate.
TRAVEL PER STEP
The linear travel movement of one full step of the motor.
HEAT RISING
Motor body’s temperature rising after certain periods running and heat exchange with the ambient.
RESPONSE PER STEP
Times takes 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
Under certain drive condition (frquency and current), the max load that the motor can drag until missing step
PULL IN TORQUE
When couples and accelerates, the max load torque (including frictions) that the motor can bear and start.
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 tensi on 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
The maximum thrust load, including shock load, that should be applied to a non-moving screw.
DYNAMIC LOAD
The maximum recommended thrust load which should be applied to the screw while in motion.
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 application
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 contains a self-lubricating material that eliminates the need for adding a lubricant to the system. The
Teflon coated screw option also lowers frication and extends life of the system.
END MACHINING OF THE SCREW (Please refer to A-40)
Standard metric or English option are available. Custom end machining specifications are also available on 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.
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 th 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)
BACKLASH
Backlash is the relative axial movement between
a screw and nut at standstill. It is normal for
backlash to increase with wear over tme. 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.
STEPPING SEQUENCE
Lead Screw Material: 303 Stainless precision cold rolled steel
Screw Coating: Teflon coating is optional
Standard Screw Lead Accuracy: 0.0045 inch per foot (0.18mm per 300mm)
Screw Straightness: 0.003inch/foot, measured as Total Indicated Runout(TIR).
All screws are carefully checked for straightness before shipment.
Screw Efficiency: From 35% to 85% dependent on lead. Also depends on the usage of an anti-backlash
nut with screw. The larger the lead, the higher the efficiency of the screw.
Operating Temperature: -20°C to + 50°C
Storage Temperature: -20°C to + 50°C , keep in dry
Screw Backlash: 0.01mm~0.1mm, depends on different lead. Anti-backlash nut should be considered for
high bi- directional positioning repeatability.
System Backlash: Includes screw, motor, and attached mechanics. This will be the sum of all the
backlash in your motion axis.
Nut Material: Polyacetal with lubricating additive; Standard is a free-wheeling nut. (Anti-backlash
version is available)
Wear Life of Screw and Nut: Depends on load, speed, duty cycle, and environmental factors (typically ≥ 5 million
cycles