Application digest

Lubricant lift lightens load

By Joe Messer, President ACCRO-SEAL Vicksburg, MI

Metal surfaces, regardless of how smooth they appear to the naked eye, are not really smooth at all. When observed under a high-powered microscope, they project a cross-section of saw-toothed irregularities. These metal-surface asperites complicate the laws of hydrodynamics in that they can poke through an oil film and cause lubrication failure.

Hydrodynamic lubrication is the ‘ideal’ situation for lubricating surfaces. In this instance, a rotating shaft generates an oil wedge, lifting the shaft from the Bearing ID.

Lubricants from ACCROLUBE^® lubricate under the mathematical laws of hydrodynamic and boundary lubrication conditions, where both aim to keep the two sliding surfaces separated by lubricating film to reduce friction and wear. Hydrodynamic lubrication occurs when a very thin film of oil separates two sliding surfaces, because a rotating shaft generates an oil-like wedge equal to the loads on the bearing.

The oil wedge, also referred to as a "Hydrodynamic Wedge," is what engineers consider a "perfect lubrication." But because hydrodynamic lubrication requires speeds under moderate loads, it is impossible to maintain these two conditions in every application.

A true hydrodynamic fluid wedge cannot be maintained under conditions of high loads and slow speeds. Under these conditions an oil film is either squeezed out, due to pressure, or the sliding motion is inadequate to maintain a lubricating film.

ACCROLUBE lubricants are targeted for conditions where a hydrodynamic fluid film will fail. The boundary lubricant media used are solids of molybdenum disulfide, Teflon^®, and graphite. The optimum of these extreme-pressure boundary lubricants is molybdenum disulfide that can handle 1,000,000 lb/sq inch in its pure powder form.

Solids used as a boundary lubricant in ACCROLUBE form a mechanical barrier that reduces metal contact between opposing surfaces. Petroleum or synthetic oils sometimes perform a dual role, acting as a carrier for the boundary lubricant and lubricating under the laws of hydrodynamic lubrication.

To speak with an ACCRO-SEAL representative, call the Lubrication Technical Dept. at (616) 649-1014 or visit www.accroseal.com

Guidelines for sizing a servo system

By Phil Micech, Product Marketing Manager General Motion Control Rockwell Automation/Allen-Bradley

When sizing a servo system it is important to understand basic performance characteristics and practical sizing guidelines. Determining specifications helps successfully size a servo system.

Understand the application. Look at the application, literally if possible, especially if you are considering a retrofit. Draw the application. Note any special circumstances. Define the machine mechanics and area in which the servo equipment will be utilized. Discuss potential issues with the vendor that could affect performance.

Size the actuator. Once mechanics are defined, size the actuator that drives the mechanics. Actuator types range from hydraulics and linear actuation to servo motors. Choose an actuator type. Define the size requirements. Consider factors such as servo motor flange, mounting hole pattern, length, shaft attributes, and mounting orientation. Take special consideration to retrofits. Determine the environment in which the servo motor will operate. Evaluate key performance characteristics for sizing a servo motor: application duty cycle and speed, and system response.

Select the feedback device. Servo motors often have an existing feedback device used for commutation and velocity loops that are inherent in the design and cannot be changed. Usually an encoder or resolver is used for this feedback. However, each possess different attributes that target them for different applications. In addition, each device could be used in the position loop with a magnetostrictive device to close the position loop. Determine if your application requires an encoder or resolver

Size the drive. In servo systems, a specific drive/servo motor combination is recommended for optimum performance characteristics such as drive velocity, current loop bandwidth, and feedback. Once the servo motor is sized, determine the specific drive for the application. Consider the following: What servo motor is used. What options and how many drives are necessary. Remember, drives are typically embedded in a control cabinet to lessen excessive environmental and temperature ratings.

Size the motion controller. The motion controller is usually external to the drive system. Today, many manufacturers are embedding the motion controller in the drive to save cost and reduce wiring interfaces, making the system more reliable and saving panel space. Define what type of motion controller to use: standalone, programmable controller-based, bus-based, open network, or CNC. Is the motion controller a stepper or servo system? Define the number of closed-/open-loop servo axes the motion controller needs to control. Review the interface from the drive and feedback devices, plus the control networks and external operator interfaces. Define the feedback device interface, if required.

For a complete guide to sizing a servo system,  visit www.ab.com/motion.