As the saying goes, "The devil is in the details." As simple as your linear motion application may seem, focusing on areas that may cause unexpected problems is highly beneficial icial. Here are a few important factors to consider when sizing a stepper motor linear actuator that will save work, money and time in the long run.
Calculating Torque
Calculating the necessary torque to move your application load is standard procedure for correctly sizing a motor to drive your lead screw or ball screw assembly. When using a lead screw, the lead and efficiency will significantly affect the required torque. As a result, you may find yourself balancing force (which will be higher with a finer lead due to the mechanical advantage of the thread) with linear speed (which will be easier to achieve with a lead screw having greater advancement per revolution).
Although this torque calculation is a significant number that will take up most of the torque budget in the system, other factors also increase the torque requirements. Elements such as inertia, acceleration, misalignment, seals, temperature and inefficiencies in the system all affect the overall torque value. Therefore, it is common to size a stepper motor for an actuator at 2x the torque requirement to provide added headroom to drive the lead screw or ball screw. If you don't account for the additional torque, steps may be lost, which can introduce lost motion or inaccuracies in the system. Although an encoder can be used to close the loop, this adds cost and takes up additional space in the design.
Besides cost and space, sizing a motor with a larger torque buffer or safety factor can result in using a larger motor or longer motor (double stack) for the stepper actuator. When reviewing the application, think about the motion required and the actuator's function in the system. For example, many designs are based on the maximum load and speed.
It's essential to ask:
- What is the duty cycle?
- Do both limits occur at the same time, or the load is lower when the speed is higher?
- Is this high load and fast speed required only occasionally or continuously?
- And are the requirements only for a small portion of the total move or along the entire length?
Over-driving
In a gripper assembly or tip eject scenario, the highest loads may only come at the end of the stroke when clamping or pushing. This is generally accomplished at much slower speeds. Otherwise, the load is much less when the speeds are higher. You don't have to size the motor for both extremes in this example. It can be sized for typical use, and over-driving can be used to generate a force of 30% or greater.
Over-driving is also a valuable tool for acceleration. More torque is required to rotate the shaft at high speed (inertia) and reach the linear speed (velocity) as quickly as possible. Ramping the motor acceleration so it is more gradual and takes a little longer to get to maximum speed can help prevent the motor from stalling.
Temperature Considerations
Temperature is often overlooked as a factor that can create issues with the performance of the linear actuators. For example, cold temperatures can cause extra drag (friction) and lost steps from the stepper motor actuator. At Helix Linear Technologies, we frequently work with engineers designing to a temperature spec of -30C and sometimes as low as -50C. It's important to consider whether the temperature is a cold storage condition, where the materials must withstand the climate and not necessarily function at this low temperature, or the application requires the motor to run continuously and reliably in these cold conditions. There are several materials to choose from in both scenarios, but they may differ with each application.
Just as important as whether the individual parts of the linear actuator can withstand the low temps, the difference in thermal coefficients of the mating parts is also a critical factor in selection. Therefore, we recommend working with a Helix Linear Technologies application engineer to address and resolve temperature concerns. Our experts help you build in additional clearance, so binding does not occur at cold temperatures and assist with the material selection so that mating parts have similar thermal coefficients, allowing them to expand and contract at the same rate.
Weight Considerations
The weight of the entire assembly or machine is also a factor that is often overlooked. Many solutions for weight reduction exist in the form of alternative materials for aerospace, military, packaging and transportation industry applications.
Aluminum lead screws are popular for lightweight applications, although they can lack strength and stiffness. At approximately 1/3 the weight of a stainless-steel screw, aluminum can significantly reduce the weight of the linear actuator assembly and the overall system -- especially when large lead screws are necessary. Titanium is another viable material option with the strength of steel and aluminum's reduced weight but comes with a higher price tag. When strength needs to be maintained, and the cost is a factor, alloy steel and stainless-steel lead screws can be hollowed out. They are gun-drilled through the center to reduce their weight significantly.
Aluminum lead screws are an excellent choice for Magnetic Resonance Imaging (MRI) applications or other applications where non-magnetic or non-ferrous lead screws are necessary. Although 300 series stainless is considered non-magnetic, it can develop slight magnetism during processing due to work hardening. Magnetic levels can be further reduced through a heat-treating process. Lead screws made from bronze and titanium are also potential options in these environments.
Custom Nut Design
Don't automatically exclude a custom nut design, as it can provide value to the application. For example, a custom nut can be designed to eliminate mounting complexity and alignment issues during system assembly. There is also added value in molding the part completely. This may require a one-time NRE (non-recurring engineering) charge but will also result in the lowest overall unit price. If molding complete can't be accomplished cost-effectively, then molding a blank with most of the nut dimensions intact and performing secondary operations is another customization option.
When designing a plastic nut, consider whether the mechanical, thermal, or other key performance characteristics are available in rod, stock, pellets for molding or both. In many cases, molding the nut makes sense and may be more feasible. When molding a nut, it's advisable to follow best practices. For example, asymmetric part shape, thick walls and lack of uniform wall thicknesses will create molding issues. We also recommend coring to reduce the chance of sink and dimensional instability. Our Applications Engineers will work with you on specifying the best material and design recommendations. We have materials to meet your specifications, whether ESD, cleanroom, underwater, food contact or space applications.
End Supports
To support the ends of the lead screws or ball screws, bearing options can include angular contact, deep groove radial, and thrust bearings. Deep groove radial bearings are commonly used to handle axial loads. The thrust load capability of a radial bearing is generally calculated as a percentage of the radial load rating. For high axial loads, consider angular contact bearings. They are designed specifically for axial loads and provide radial support for the lead screw as well. Thrust bearings will take the highest loads but generally don't support the shaft radially, so they are generally used in combination with a radial bearing or other means to support the shaft.
A caution about radial bearings: the static load rating is typically lower than the dynamic load rating. You will want to watch for this when sizing a bearing. It's important to determine what the maximum load will be when the lead screw is rotating (dynamic) or when it is stationary (static).
Seals are available for many bearing types. Sealed bearings can help keep out dirt and dust and extend the life of the bearing, especially in harsh conditions. However, it's imperative to check how the addition of seals impacts the specifications. In some cases, seals can lower the load rating. If seals are not available for a particular bearing type, bearing mounts are available with seals built in to shield the bearings from contaminants. Whichever bearings you select, the lubrication in the bearings may have to change depending on the conditions. For example, cold temperature exposure, outgassing concerns, high loads, and other special situations may warrant changing the standard lubrication to grease that is more suitable for the application.
Helix Linear Technologies Application Engineers Can Help
Physics and logic will drive different decisions made during the design cycle. Often, there is a balance between the optimal selections and other factors like price and lead time. Many elements may also differ during prototyping and production. Whatever the case, A Helix Linear Technologies Application Engineer, is here to assist you with the various choices.