Lead screws use the helix angle of the thread to convert rotary motion to linear motion. The performance of a lead screw depends on the coefficient of friction between the nut and the screw, which in turn depends on the material used for the nut and screw.
Here’s an overview of the most important factors to consider when selecting a lead screw for an application.
Begin by looking at the required load capacity. Plastic nuts are typically used for light loads of less than 100 lbs, although plastic nut designs for 300 lbs and beyond are possible. Bronze nuts, on the other hand, can be used for applications in excess of several thousand pounds.
The pressure-velocity, or PV factor, is the product of the pressure and velocity between the nut and screw. It helps determine the load, speed and duty cycle that the nut can handle. Plastic materials have an intrinsic PV rating, the point at which frictional heat causes permanent deformation of the plastic. So the more load applied to a lead screw assembly, the slower it must be turned to avoid exceeding the nut’s PV limit. Likewise, the faster the nut is turned, the lower the available load capacity. The primary modes of failure for plastic nuts are wear and PV. By designing within the PV envelope, failure occurs as a result of wear. Wear failure occurs more slowly than a PV failure and can be accounted for through application of case studies and testing.
The efficiency of lead screws typically ranges between 20 and 80% and depends on its helix angle. As a general rule, higher helix angles mean higher efficiency. A higher helix angle is more efficient because less of the energy used to drive the lead screw goes into overcoming friction. This is because the number of times the screw must rotate to achieve the same linear displacement is reduced on a high helix screw. A disadvantage of a high helix angle is that more torque is required to turn the screw.
It’s important to note that when the efficiency of a lead screw exceeds 50%, the lead screw becomes back-drivable, which means it can be driven backwards by the load. Back-drivability is a disadvantage in many applications because a brake may be needed to support the load if the lead screw is mounted in a vertical orientation. On the other hand, back-drivability is a plus in some applications.
Lead screws come in a wide range of leads, from under 0.050 in. to 2.00 in./rev or more. The use of a wide range of leads can deliver a wide variation in jog speeds up to 70 in./sec. This lead screw feature can provide advantages in many applications. For example, devices that need to be positioned with a high level of accuracy can use a lead screw with a low helix angle to obtain high positioning resolution. Other applications benefit from fast jog speeds and low screw rpm, providing quiet operation and long life. The maximum rpm of a lead screw is limited by the critical speed of the screw (speed at which resonance occurs). Lead screw nuts can be driven at high rpm, but depending on the load applied, heat buildup may occur and limit duty cycle.
Plastic and stainless steel lead screw assemblies are typically limited to a duty cycle of 50% under the rated load. Lead screw assemblies that use bronze nuts have higher load capacities, but these heavier loads increase frictional heat so their duty cycles must be lower, often as low as 10%. This can be calculated more accurately using the PV factor. Lead screws can operate at 100% duty cycle at light loads and moderate speeds, or they can run at lower duty with either high load and low rpm or low load and high rpm, but not both. High load and high rpm relative to a given thread size and nut design will result in overheating and failure.
Backlash is another significant consideration in applications where positioning accuracy is important. Standard lead screw configurations typically have backlash ranging from 0.002 in. to 0.010 in. Anti-backlash lead nuts eliminate freeplay and increase repeatability. Most designs place a compression spring or other compliant member between two nut halves to remove radial clearance. Preload between the nut halves must equal or exceed the applied axial load in the direction in which the assembly is loaded through the take-up mechanism to prevent lost motion. The result is a higher torque requirement which typically necessitates a larger motor.
There is no reliable closed-form solution for calculating the life of a lead screw because friction wear can be non-linear. A single wear coefficient is often insufficient to predict performance throughout the life of a lead screw assembly. Life is estimated by case study under controlled operating conditions. In the case where the combination of loading, speed and duty cycle exceeds a certain limit, failure is predictable and will occur rapidly. For applications that operate near the recommended design load at greater than 500 rpm, check with the engineering department of your lead screw supplier. They can calculate the PV factor, which is a key design factor in proper sizing and selection of lead screw assemblies that use polymer nuts.
Lead screw mechanisms using bronze nuts also need a lubricant, usually a thick damping grease. Lead screw assemblies with plastic nuts can run well without lubricant due to the internal lubricants in the nut material, but the use of a gel type lubricant will help increase allowable loading and extend life by reducing friction between the nut and screw. If particulate is present, the screw should be cleaned before re-applying lubricant. Re-apply when there is no visible film remaining on the flanks of the screw thread.
Grease should not be used in environments with significant particulate or debris that can load the grease and cause it to become an abrasive slurry. In this type of application, dry film lubricant should be used instead. PTFE coating is a dry film that creates a lubrication barrier between a metal substrate and a polymer bushing or lead nut. It is well suited for use with plastic nuts and stainless steel lead screws. Lubrication maintenance intervals can be eliminated and the coating does not attract particulate like a gel lubricant.
Online selection and sizing
Online tools can also help reduce the time it takes to identify a suitable product for a specific application. One example is Linear MOTIONEERING® (www.linearmotioneering.com) from Thomson. Users enter key parameters such as mounting configuration, loading conditions, life requirement, orientation, speed and stroke.