ARTICLES

Heat is Death #1:
High Oil Temperature Degrades Lubricant

Excessive heat and high temperature can kill or drastically shorten the life of machinery and equipment. Here is an example where a variable speed gearbox drive for a tunnel oven had problems for years.

The variable sheave gearbox drives the slow moving metal wire band conveyor through a 120′ tunnel oven with product (hearth bread) on the band. The band is hot, 300 F to 450 F, and surrounding equipment are exposed to the heat source. The gearbox assembly consists of a motor driving a variable sheave drive that is coupled to a larger reducer gearbox coupled to the band drive roller. The variable drive (made by P.I.V.) uses a special metal link belt and taper-side variable sheaves with oil bath lubrication. The sheaves are adjusted by a servo motor to change speed.

Problem: The metal belt was constantly wearing out every three or four weeks. It should last a year.

First Solution: replace the metal sheave belt when noisy, typically every three weeks, on shut down weekends. Replacement requires about 5 skilled mechanic hours and $300+ materials. This works. Downtime eliminated, but “do-or-die” belt replacement must be performed.

But wait a minute: at the bakery, there are two almost identical tunnel ovens, same make, same gearbox setup, almost twins except one is shorter. The short oven has the same variable sheave and the rest of the equipment is a scaled down version of long oven by about 30%. The metal sheave belt on the shorter oven has a normal life span, at about a year. What is the difference that causes one to wear out every three weeks and its near twin to last a year or more?

Cause Found: the gearbox in the long oven is hot, about 230 F on top of case, where short oven gearbox is cooler, less than 170 F on top. Oil lubricity properties degrade above 180 F per oil manufacturer. Gearbox manufacturer emphatically requires proper lubrication of gearbox. Think about it: metal belt and variable sheave system is a high friction operation with belt links constantly sliding against each other and the metal sheaves while under load. Must have oil film, or friction will prematurely wear out system.

Why hot: the reason one drive is hotter than the other is the configuration of the sheet metal enclosures. The enclosure for the short oven gearbox is tighter, further from the head roller, and the path for hot air to enter is more restrictive. The enclosure for the long oven gearbox has a lot of gap on its side next to the oven and is close to the oven. Hot air drafts through the enclosure, like a chimney. In so many words, the enclosure is connected to a hot air furnace. In a bakery, equipment must be enclosed for sanitation to keep out flour dust and keep potential contaminants in, so removing enclosure is not an option.

Final Solution: Close up the sheet metal a little better and install a small propeller vent fan (about 8″ diameter) in side of enclosure to push room air in and keep the enclosure slightly pressurized. The enclosure could not be totally closed up, so the slight pressurization pushes room air out of the gaps and keeps hot air out.

Result: Temperature on top of gearbox drops below 180 F and variable sheave metal belt does not wear out. Frequent “do or die” weekend metal belt replacement eliminated.

What about high temp oil? High temperature oil could be used, but that is dancing around the real problem……gearbox is not for high temperature service and it is too hot. High temperature can affect seals, gaskets, other materials, clearances, and oil in any gearbox, whether the large gearbox driven by variable sheave unit, or the variable sheave unit. The variable sheave unit is more likely to suffer from oil breakdown due to its high friction operation.

 

 

Heat is Death #2:
High Room Temperature Contributes To Bearing Failures

Basic simple anti-friction (ball, roller, needle) bearing units do not like high temperatures unless made special. If temp is too high, the grease, a must have, loses its viscosity and other properties resulting in lack of film, increased friction, further increase in temperature and drastically reduced bearing life, if not complete failure. Here is an example from the trenches:

PROBLEM-FAILING CONVEYOR BEARINGS: A new state-of-the-art high-speed loop slitter was installed and operated perfectly, except conveyor bearings in upper sections kept failing or approached failure while rest of same type bearings were fine. What is a loop slitter? Its 200′ of belt conveyor in a big oval loop standing about 30′ high, 100′ long, and about 8′ wide used to “slit” thin web product from the inside of a loop of urethane foam slab that starts out at about 200′ circumference with 4′ high by 6′ wide cross section. At end of run, only the bottom 3/4″ is left after the rest is converted to web, rolled up, and discharged from machine. Pretty fantastic! Running at high speed is critical for high production levels of web product (3/8″ to 5/8″ thick foam sheet)…that is why big bucks was paid for machine.

EQUIPMENT BACKGROUND: The machine manufacturer only makes superior quality equipment…….something else must be wrong. The machine was one of the first of the new high-speed generation, so design improvements may be in order. Access to the upper conveyor sections is extremely difficult due to lack of access platforms and extremely limited floor space for lifts, etc. The bearings in question are idler rollers that support the conveyor belts. There at least 20 on machine. They have typical sealed ball bearing units made by a quality manufacturer. Replacing with an upgraded bearing requires significant effort to replace or modify. A high temperature, or no-grease (self-lubricating material or special plastic), or higher capacity bearing with same dimensions is not available. Moderate to major modification of each roller (new roller design, modification of existing rollers or fabrication of new rollers, potential modification of bearing support in conveyor frame, etc.) is required if different size bearing

Mechanics servicing the noisy and grinding bearings in the upper section say they are dry with no grease. Need grease. They are using grease gun with needle nozzle to push grease into sealed bearings. Not a good method, but necessary for survival. They want to modify to install grease-able bearings and add remote grease tubing to accessible station where grease could be applied remotely without “climbing” on machine frame. As stated above, installing different bearings requires substantial modification.

CAUSE-HIGH TEMPERATURE ENVIRONMENT-POSSIBLE LOW DESIGN SAFETY: What is the difference between the bearings in the lower section that don’t have problems and the upper section that does have problems? Difference appears to be operating environment temperature. The machine is tall and resides in semi closed room with a penthouse to allow space for the upper section. Ventilation in room is very poor. No ventilation in upper room section and penthouse. It is dead (stagnant), hot air. The machinery produces heat and heats the air. Hot air rises and is trapped in the upper sections of the room and the penthouse. Building is not air conditioned. In summer, interior temperatures, at floor level, are 90 to 98 F. Thermometer in upper part of room shows 105 to 110 F where conveyors operate. Overall, the temperature in the penthouse is at least 15 to 20 F higher. Since the machine is a new generation, it is possible that not enough safety was designed into the bearings. Other factors are high speed and conveyor loading. Speed-is-death acts along with heat-is-death in many applications. The bearings may be at or near their limit for speed and load with higher temperature pushing them over “the edge”.

SOLUTION-EXHAUST HOT AIR: pull hot air out of room high point and let cooler air from below take its place resulting in cooler environment for upper conveyor sections. A properly sized exhaust fan was installed in roof of penthouse to pull air out. Temperature became nearly same for lower and upper sections. Bearing problems, relative to lower section bearings, significantly decrease or disappear. No equipment modification required for improvement. Bearing design is still an issue, but at least there is some relief.

LESSON: think about environment for equipment before installing. This was done to some extent because fan support frames were installed in penthouse roof structural steel along with runs of power wiring conduit anticipating future need. Company management frowned on exhaust ventilation fans though others disagreed. I believe the sealed bearings probably did not have enough safety built in for running at high speed and tolerating higher than normal room air temperature. Hot dead air probably pushed the bearings over “the edge” into overheating and degrading their grease. Bearing bottom line: a better, higher capacity sealed bearing is probably needed on next machine version and upgrading existing bearings may be worthwhile.