![]() These variables can be managed by selecting mechanical components that reduce friction and improve efficiency. The types of materials that slide across each other can impact the friction coefficient as well as whether those surfaces are lubricated or unlubricated can ultimately influence the friction torque. For example when designing a linear actuator selecting a leadscrew or a ball screw can have a significant impact on friction torque. The mechanism chosen can affect friction torque. There are several factors related to the load mechanism that have an effect on friction torque. If the acceleration is an unknown a good rule of thumb for calculating the acceleration is to use a trapezoidal motion profile where one third of the move time the motor is accelerating, one third of the time the motor is running constant, and one third of the move the motor is decelerating. ![]() When there is flexibility in the design it is best to use the lowest acceleration possible thus lending to the lowest acceleration torque. The higher the required acceleration of the load the higher the acceleration torque and thus a higher maximum motor torque. Another reason why knowing the motion profile is critical. The maximum torque is the sum of the acceleration torque and frictional torque. Once we have the inertia calculated we need to calculate the maximum torque of the system. This is why having motor torque curves handy when selecting the motor is a must. This does come with a tradeoff of higher motor speeds to maintain the same output velocity. Often times minor changes to a leadscrew pitch or adding a gearbox will provide the desired effect of lowering the inertia ratio to the desired value. When the inertia ratio is higher than 10:1 changes to the mechanical system need to be considered. If high performance, high accelerations are required a ratio of 2:1 or 1:1 may be more appropriate. In some instances it can prevent the motor from ever getting motion started.Īn inertia ratio between 5:1 and 10:1 is a good guideline for most systems. If the ratio between the load inertia and motor inertia is too high system performance will suffer. The load inertia is critical in overall system performance. Each and every component of the mechanical system has inertia and must be included when determining the load inertia. By definition the moment of inertia is a measurement of how difficult it is change the rotational velocity of an object. A parameter that is sometimes overlooked is the load moment of inertia or simply put load inertia. To properly size a motor you will need to calculate both torque and inertia. Understanding requirements such as resolution, position holding, and stopping accuracy may direct the designer to one motor technology or another such as stepper or servo. It is at this step when other specifications or parameters need to be reviewed. Having that completed design will also assist when gathering the information that will allow the correct sizing of the motor. How long is the move cycle? Is there a dwell between moves? What is the distance of travel? To understand these questions often times requires the mechanical system to be designed to completion or near completion. This step can sometimes be the most challenging. However if a few simple steps are followed the required information can be gathered that will provide direction to the best fit for the application. ![]() In the past this may have been avoided because it was considered difficult or too time consuming. That means no longer using overkill or “it’s what we did last time” as a design criteria. As the variety increases, the need to correctly select the motor solution is becoming critical to ensure the most cost effective design while guaranteeing a reliable and well executed project. Engineers today have an ever growing selection of motion control products to choose from when designing a system. ![]()
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