Motion System Design

Design by Objective: High Speed

In today's world, faster equals better. With the need for speed increasing, the editors of Motion System Design asked industry experts for application tips and guidelines when designing a high-speed motion system

Define “high speed.” What is considered leading edge today?

Anthony/Timken: High speed, referred to as dN, is calculated by multiplying the bearing bore diameter in millimeters by the shaft rpm. If the solution to this equation is greater than 1 million, it is often considered highspeed.
High speed in linear motion depends on the type of linear motion product. In terms of ballscrew drives, NSK has a 50-mm-diameter x 100- mm-lead ballscrew capable of 7 m/sec. However, it’s unlikely anyone would drive a ballscrew this fast. For linear motor drives, 5 m/sec seems to be the fastest practical speed. And belt-driven actuators are limited by the maximum allowable speed of the linear bearings. For linear guides, the maximum allowable speed is 5 m/sec.
In the world of linear devices driven by ballscrews, high speed motion is 60 m/min and higher, with acceleration of 1 g and greater.
In terms of the rotational speed of squirrel-cage induction motors, or PM brushless motors, high speed is when rotor surface speed exceeds 75 m/sec. While motors can certainly be designed to run with higher rotor surface speeds, it might mean trade-offs in cost, complexity, and reliability.
High speed is a relative term, and when dealing with motors, one missing part of the equation is frame size. While 20 krpm wouldn’t be considered high speed for a motor with a diameter less than 1 in., it would be fairly high speed for a motor with a 4 in. or larger diameter.

In what applications is speed of greatest importance?

Karl/Bosch Rexroth: In general there are productive and nonproductive times in machine cycle time. Any motion in the non-productive time doesn’t make money, so movements must be made in the shortest possible time.
Speed is of great importance in metal cutting, medical, and aerospace applications, where high speeds are required for accuracy.
In high-speed dental hand pieces, material removal in cavity preparation requires precise cutting with an extremely high impact force because of the hard surface of tooth enamel and dentin. Burr design and speed help provide a more controlled impact for material removal, as opposed to applying a shear force as in most cutting operations.
Aubrey/igus: Speed plays a pivotal role in high-duty cycle applications where production numbers and capacities are determined by the speed at which the machine can operate. The faster the machine moves, the more it can produce and the more money your organization can make. Simply put: higher speed means higher throughput.
Nick/Oriental Motor:
Speed is a factor when throughput is a major concern. In large machines like PCB inserters, the distance to be traversed can be long, so the time it takes to go from one end of the machine to the other is important.
John/alpha gear:
Production, converting, and printing are key industries where speed is important. Slight speed increases, without a reduction in accuracy, can result in additional products being manufactured.
With applications like press feeders or cut-to-length operations, speed should be looked at in the context of “speed of response.” These applications are challenging, requiring multiple accel/decel cycles per second. In the motion control world, speed of response is often measured in terms of torque-to-inertia ratio, or even as velocity- loop bandwidth. Another aspect of speed of response is how fast a process line can be changed from one product to another.
George/NSK: Speed is important in longstroke applications. Speed and acceleration requirements are often confused, and it’s common for machine manufacturers to publish a maximum speed capability the machine would never be able to reach because of acceleration limitations.
Speed is important in scanning. High-resolution scans over a large area can take significant amounts of time if the data-capture and triggering rates are slow. Using high-speed data acquisition and increasing communication speed between the motion control and I/O can help reduce the time it takes to do the scan.

What are the limiting factors in a motion system when trying to maximize speed?

Bill/MEI: The actual mechanical design of the system and available power to apply to the problem are limiting factors. Something stiff, but heavy, may require more power than available to move quickly; while a light and flimsy piece won’t be able to move quickly and maintain accuracy at the end of the move.
In terms of speed of response, inertial effects are obstacles. In the context of absolute speed, limiting factors include speed-dependent losses, such as friction, windage, or motor lamination core losses. Vibration can also be a substantial issue; an amplitude of vibration only a fraction of a mil peak-to-peak can be excessive when it happens once per revolution at 10,000 rpm.
One constraint when operating motors at high speeds is switching losses, whether electromechanical or electrical. Mechanical limitations are also constraints. In a typical brush motor, a major concern is keeping the winding wire contained in the slots. For brushless dc motors, the concern is keeping the magnets from leaving the motor shaft.
John/alpha gear: Heat, vibration, control, and reliability are the limiting factors of servosystems. Speeds as low as 1,500 rpm can generate enough heat, due to the small surface areas of today’s servoplanetary gearheads, to prematurely wear oil seals and cause leakage.
Limiting factors are operating loads, heat generation, and machine life. It’s important to address these factors through bearing design, lubrication, and heat dissipation.
The biggest limiting factors for small precision bearings used in dental hand pieces are particulates from contamination or components in the material used to gel a grease. The retainers that secure the rolling elements in the bearings can be damaged when their strength is compromised by incompatible oil or other fluid or by contacting particles in operation. At high speeds, any solids in a bearing raceway can cause bearing failure.
Besides the limits of critical shaft speed and maximum ball velocity, maintaining adequate lubrication is a must. Higher speeds can create excessive friction and heat buildup in the nut.
Heat can degrade the sliding elements causing lower performance, and friction has a direct effect on the temperature range, which affects speed. Reduce friction and heat by using different bearing/shaft combinations.
One of the biggest limiting factors in automated machine control applications involving both motion and I/O is the rate of communication between the two. To overcome it, use high-speed data-acquisition boards in combination with a motion controller that can generate triggers at a high rate. The easiest way to do this is to use communication protocols that don’t require special wiring.

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What should engineers know about component interactions and how they affect speed?

Karl/Bosch Rexroth: For positioning at high speeds, remember applications with short travel and high speed require high acceleration. Machine structures must be able to handle the acceleration changes. The OEM must use FEA to determine resonance frequency locations and make the design so frequency nodes are few and amplitudes are minimal.
In the case of a motor being run at high speed, support stiffness, whether flange or foot mounted, is critical to avoiding system resonance. Additionally, running at high speeds often requires more frequent lubrication and, in some cases, specialty lubricants.
Nick/Oriental Motor: Sometimes top speed of a motor may be greater than the maximum speed for the attached mechanism, a leadscrew, for example. If the two components aren’t specified together, leadscrew damage or reduced life may occur.
Understand the interrelationship between electrical and mechanical systems and what is expected from the actual application. Performance should be specified and designed for at the beginning, not as an afterthought.
The higher the speed, the less room there is for contamination to damage bearing components and cause failure, or geometric variance that may be caused by seal or retainer swelling or shrinkage.
Ballscrews are typically supported at one or both ends by rotary bearings. Using preloaded precision duplex angular contact bearings on both ends increases a screw’s critical speed rating, enabling higher speed operation.

• Choose linear guideways compatible with the required speeds.
• Ensure proper alignment of the guides with the screw to reduce any side forces on the ball nut.
• Unless motor shaft is exactly in line with the ballscrew, select flexible couplings to connect the two.
• Make certain the lubricant is suitable for the desired speeds.

What pointers can you provide to help engineers achieve maximum speed in their design?

Mike/Reliance: In the context of speed of response, the ideal situation for transferring rotational energy is to match, as closely as possible, the reflected load inertia to the motor inertia by adjusting the speed ratio.
Understanding the requirements at the beginning of the design cycle is key to a successful design. All engineering disciplines need be involved. If all parties work together from the beginning, the number of redesigns and late projects is reduced.
In ballscrew systems, pay special attention to critical speeds for longer strokes.

• Keep the system well lubricated.
• Make sure machine structure is stiff enough for the higher acceleration loads and vibrations that will result.
• Remember settling time increases with speed.

Andy/Rockford: The factors that determine the speed on a ballscrew shaft are lead, root diameter of the screw thread, and bearing support rigidity. The larger the root diameter and the faster the lead of the screw, the higher the speeds.
Proper use of IEC 61131-3 standard programming languages will result in faster applications.

What is the most impressive accomplishment in terms of high speed your company helped make possible?

Anthony/Timken: Timken’s bearings enabled the operation of a turbine with the smallest available footprint for electric power generation at the point of consumption. Some of the requirements included high reliability, longer operating service life, ultrahigh speed, continuous operation, high radial and centrifugal loads, and reduced bearing dimensions.

The operating speed and load requirements were achieved, but most importantly, the service life was extended by a factor of 10 compared to standard bearings.

Early involvement of the system manufacturer, component providers, and Timken bearing engineering team was the key to success. This allowed for different analytical models and iterations prior to the prototype phase and an ample amount of testing.
NSK designed a 50-mm-OD x 40-mm-lead nut-rotating ballscrew with a hollow shaft and special damping rod for a large router with an X-axis stroke of 5 m. For a standard ballscrew, the rotational speed would have been limited to around 500 rpm. However, using NSK damping technology, speeds in excess of 2,000 rpm are possible.
There is an application currently using our linear guide system that has performed more than two million cycles with no measurable wear at 150 in./sec. It requires extreme precision and operates at a very short stroke. The key was the sliding plastic bearing and hard anodized aluminum shaft, which creates the best material combination for both speed and performance.
We have ballscrews operating at nearly 3,000 in./min on a continuous basis in the glass manufacturing industry. Achieving such speed requires a rugged leadscrew with a customfitted ball nut. It’s also necessary to use the proper lubrication.
John/ELAU: Most recently, we overcame the mechanical constraints of installing servosystems on rotating machine members. Rotary liquid filling, capping, and labeling machinery have defied servo automation due to the complexity and space restrictions of mounting servomotors and drives on rotating carousels.

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