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. Please also reference the Glossary (page 85 of the catalog) to support your understanding.

  1. Some basic design considerations are as follows:
  2. What is the load of your system?
  3. What is the required speed to move from point A to point B?
  4. What is the distance to be travelled?
  5. What is the required time to move from point A to point B?
  6. What accuracy does your application require?
  7. What repeatability does your application require?
  8. Horizontal vs vertical orientation?

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 effi cient at power transmission.

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 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

Specifi ed 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.

technicalinfo_radialloading2.png

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 efficiency 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 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:

  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 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

WHY CHOOSE ONE FORM FACTOR OVER THE OTHER?

  1. What is the best mechanical fit for your application?
  2. How do you plan to attach the screw?
  3. Is rotation of the screw acceptable?
  4. Does your application require an encoder or brake?
  5. What is the stroke of your application?

WHAT ENVIRONMENTAL CONSIDERATIONS DO YOU HAVE?

Koco Motion / DINGS’ linear motion systems are designed to operate in dry and non-corrosive environments. Standard products do not have an IP rating. Operating non-IP rated linear systems in dirty or corrosive environments will significantly reduce product life.

TEMPERATURE

Very high or low temperatures may cause significant changes in nut fit or drag force.

MAXIMUM DYNAMIC LOAD

Each NEMA frame size motor has a mechanical load maximum that should not be exceeded. For more information, see Speed/Force curves for the individual frame sizes.

MOTOR SELECTION

In order to select the right motor/lead screw combination with the lead screw, several factors should be considered:

  1. How much force is required?
  2. What is the desired step angle?
  3. Detent or holding force requirements?
  4. Physical size restrictions?
  5. What type of driver (amplifier) are you using?

Product Selection Guide

To reduce complexity and cost of a design, it is important to accurately size a motor/lead screw combination. Below are a few simple steps in selecting the necessary components for a given application.

Step 1 – Choosing a motor NEMA size (Force requirements)

Here is a general overview of the output thrust vs. NEMA size:

As the NEMA size of the motor is increased, the output thrust of the actuator is consequently increased.

Step 2 – Choosing a screw lead (Force and Speed requirements)

After estimating the required thrust and choosing a NEMA size that may fit your application, the speed and acceleration of the load must be considered and evaluated to choose an appropriate screw lead.

Due to the nature of lead screws, the output speed and output thrust achievable by a motor/lead screw combination are two inversely proportional variables (i.e., increasing the required thrust will lower the achievable speed for a motor/lead screw combination). Therefore, the maximum output force of a system is lowered for applications that require higher speeds.

For complete motor/lead screw selection data, please refer to the speed/thrust curves for each NEMA size.

Although these two steps provide a solid foundation in motor/lead screw selection, other variables must also be considered:

  • Duty Cycle
  • Desired Life of a System
  • Environmental Considerations
  • Positional Repeatability
  • Acceptable Backlash
  • Acceleration/Deceleration Requirements
  • Driver Specifications
  • Vertical or Horizontal Orientation

Because of the numerous variables involved in motor selection, it is highly recommended for users to proceed with physical testing to accurately determine the motor/lead screw combination required for a given application.

NOTE: Although this section aims to provide a rough guide to selecting a motor/lead screw combination that best fi ts an application, we recommend contacting our application engineering staff for further assistance with the motor selection process.

Trapezoidal Move vs. Triangular Move Profile

There are generally two widely used movement profiles. Depending on the required travel time and distance, different movement profiles can be used. The area under the curves below is the minimum stroke of the linear actuator, its required travel.