Don’t Procrastinate…Innovate!: Minute Maintenance, Part 2

25 March 2014

Ken Bannister, Contributing Editor

Let’s recap from Part 1 (pgs. 12-13, MT&AP, Jan. 2014): Minute Maintenance is the pursuit and implementation of innovative proactive maintenance methods, processes, techniques and tools designed to reduce or eliminate non-value-added (waste) maintenance activity to produce an efficient, effective maintenance result measured in minutes, using a lesser skill-set requirement than that needed to perform a repair.

Major interventions (i.e., overhauls and repairs) require time and high skill levels, and almost always result in major asset dependability (reliability, availability and maintainability) and production-downtime losses. Minimizing loss (waste) is achieved by understanding and identifying failure onset in a timely manner through the recognition of an asset’s current condition compared to its optimum condition so that only minor intervention is required to assure asset dependability with little or no downtime loss.

As an asset progresses through its life cycle, it can be subjected to many external influences that can cause and/or accelerate component failures. These influences include temperature (heat, cold), vibration, contamination (water, dirt), neglect and abuse—all of which conspire to produce premature wear and a corresponding requirement for major maintenance. Recognizing and managing these influences can significantly increase an asset’s reliability and service life.

Machine designers are aware they have little control over the external influences or “ambient condition factors” to which their equipment will be subjected. To compensate, they design equipment with consumable devices intended to act as part of the machine system and, more importantly, as a “tell-tale fusible link” to protect major components and systems. We know these “devices” as lubricants, filters, belts, couplings, fuses, adjustment/calibration mechanisms, and others. Intended as the “weakest link” in the design, they’re simple to assess and correct and, as such, must figure in our proactive minute-maintenance approach. In turn, we can build a simple and effective preventive maintenance-check program around the referenced consumable devices and external influences. Consider the following V-belt example.

A motorized, belt-driven fan unit
V-belt drive systems transmit power at a defined number of revolutions per minute (RPM) from a motor-driven “drive” sheave pulley to one or more “driven” sheave pulleys attached to, in this case, a fan shaft.

A V-belt is designed to “wedge” itself into the v-shaped sheave groove and ride with full belt-side contact at the top of the groove, leaving a substantial gap between the bottom of the belt and the groove valley. Worn or non-matched belts that ride lower in the groove (known as differential driving) can eventually bottom out and polish the groove valley and should be replaced quickly.

To transmit power efficiently, one of the sheaves employs an adjuster mechanism to allow the belt(s) to be tensioned to a point that under load will “slip” between 1% to 3% and permit intentional creep and release from the wedged position in the groove as the pulley turns. If tension is below 1% (too tight), the belt won’t release correctly and, consequently, generate frictional heat; if over 3% (too loose), the belt will slip too much and start to “dance,” creating rubbing friction in the sheave, raising the temperature and causing premature wear of both belt and sheave.

Checking for slip is simply a matter of using a handheld strobe light to check and calculate the RPM speed difference between the driver and driven pulley. For example, if a 1750-RPM motor is used with 1:1 ratio sheaves, the driven pulley should be running between 1700 and 1730 RPM when tensioned correctly. If not, it’s in a no-go state requiring immediate attention. A correctly tensioned belt running on an unworn, correctly aligned sheave pulley is designed to return an operational efficiency close to 97%.

Most V-belts are manufactured from an elastomer that encases longitudinal rows of polyester or Kevlar internal-tension members. During power transfer, belts are subjected to fatigue-causing stresses that eventually lead the belt-tension members to fail. But, provided the belts operate at a temperature less than 120 F and are installed correctly, they can be expected to deliver 15,000 hours or more of belt-life.

High operating loads with large fans and motors require multiple belts to transmit power with minimum energy losses. These belts must be matched if they are to be tensioned successfully. Matched belts are often purchased in sets of two, four, six, etc., that are manufactured from the same batch of rubber. If a system is designed for six belts and only five are used, it will be under a high operating load and surpass the belt-load design factor—leading to overheating, inefficiency and premature failure. Belts should be visually inspected regularly to ensure all are in place and that they are matched.

Misalignment, in both offset and angular form, is a major problem with belt-driven systems. It causes a belt’s tension members to flex sideward and vibrate, creating additional stress. When a misaligned belt enters into the sheave groove, it “rubs” the sheave wall, raising the belt temperature through frictional heat that results in rapid wear of both belt and sheave. Precision alignment of driver/driven systems using laser or reverse-dial methods is a must to reduce heat, wear and energy loss. Sheave wear is easily checked using a $10 sheave profile gage. The “tooth” profile is placed in the sheave groove and a flashlight is shone from behind. If more than 1/32” of light (wear) is evident, a no-go state exists, requiring replacement of the sheaves and belts.

Once the motor and fan are aligned and all fasteners torqued correctly, the driven system will run quietly with virtually no vibration present in the motor. When this is achieved, the motor fastener bolts and frame tension adjuster nuts can be line-painted in position, with a check line across the fastener onto the fastener plate. If the fastener becomes loose and slackens off, the painted lines will not align. This will indicate a no-go state and quickly allow the problem to be noticed and arrested.

Taking this approach and building a first-alert PM based on the equipment’s weakest link helps us compile a checklist like the one shown below, that will identify, in minutes, a no-go state (exception-based maintenance) requiring a skilled intervention.

Minute Maintenance: Belt-Drive Assembly Checklist

•  Check the check box against the task only when a no-go exception is found.
•  Using a strobe light, check that driven pulley speed is between 1700 and 1730 RPM.
•  Using an IR thermometer or camera, check that each belt temperature is < 120 F.
•  Check that all motor painted fastener alignment check lines are aligned.
•  Check that there are no dancing or heavily vibrating belt(s).
•  Check that all belts are matched for size and batch numbered.
•  Check that all belts sit in a similar position, flush with the outside sheave diameter.
•  Check for smell of burning rubber.
•  Check for visible signs of abraded rubber around the sheave pulley.
•  When machine is not running, open machine-guard inspection window and check all sheave grooves using a groove-profile gage and flashlight for <1/32” wear (LOCKOUT required).

Performed on equipment that is running (with exception of the last task), this simple-objective checklist can be completed in less than 10 minutes, by a minimally trained non- or semi-skilled individual. Any no-go finding is to be immediately acted upon by skilled personnel. Good luck! MT&AP

kbannister@engtech.com