Aluminum Furnace Maintenance Tips

Hot Cars

HIGH TEMPERATURE WHEELS

Many furnaces have cars that go into the furnace. Hot cars have wheels and bearings that are designed to take furnace temperatures up to 1200°F. The wheels are usually forged steel or cast iron or steel.  The shaft is stainless steel. The rollers in the bearing are made from special die steels that hold contact surface hardness under high temperature service.

The two most important factors for keeping the load car wheels in operating condition are avoiding mechanical damage and proper lubrication.

The furnace car structure, the number of wheels, and their location is designed for a specified load pattern and capacity. Mechanical damage to a wheel will occur due to overloading of the car, dropping a load on a car, or pushing the car sideways on its track when loading. Obstructions on the rails or in the rail groove can also break a wheel casting.

For lubricating high temperature wheels, dry lubricants are recommended. Several types of dry lubricants are available, but best results have been obtained using powdered graphite. The lubrication procedure should be performed with extreme caution, as it can produce a flammable combination of materials.

The car should first be removed from the furnace, and allowed to cool. The powdered graphite is placed in a liquid carrier, which must evaporate completely, leaving the solid lubricant in the bearing. A mixture of powdered graphite and kerosene consisting of 5% by weight of graphite and 95% by weightof kerosene should be mixed in small quantities (about 1 month supply). After thoroughly mixing the lubricant, it is to be poured into the wheel oil cup with the graphite still in suspension.

When lubrication is added, excess lubricant must be noticeable running out of the wheels. The excess lubricant flushes old dry graphite out of the bearing, preventing a buildup, which has been known to jam the rollers. Lubricate the bearings only when the wheels are cool enough (125°F or lower)not to boil or flash the kerosene. This could start a fire, or prevent the needed flushing action.

The wheels are shipped without the dry lubricant. Only oil for corrosion protection was applied. The wheels are mounted correctly and checked prior to shipment. The initial lubrication of the wheels should be made with the car hanging from a crane, or with the wheel in the door gap, so the operation can be observed from below. Check all mounting bolts and alignment of the wheels. Turn retaining washers by hand and check for clearance. Lubricate and run the wheel by hand until lubricant is running out both sides.

During furnace operation, check the wheels after each cycle to ensure that all wheels are turning freely. The frequency of lubrication depends upon the furnace operating conditions (wheel loading, furnace temperature, etc.). Some users have minimized their maintenance by lubricating all wheels between each furnace cycle. In other instances, lubricating the wheels once a week is sufficient. Over-lubrication of the wheels must be avoided, since the excess lubricant has been known to jam the movement of the wheels.

If excess graphite or dirt is allowed to buildup inside the bearing’s caged rollers, they will jam, twist, and will cause the wheel to traverse sideways, which will lock it up against the side thrust washer. Once this occurs, the only solution is to take the car out of production, dismantle the wheel, clean all residues from the bearing, straighten the bearing cage, and re-assemble the wheel. For this reason, we suggest over lubrication of these bearings be avoided.

There are commercially premixed graphite lubricants available, but some of these contain additives, which will leave an unwanted residue behind after heating. Like excess graphite, this residue can cause the wheel to jam. The graphite lubricant must always be applied with a carrier that is completely residue free after the wheels are heated.

If bearing trouble is suspected, the wheels may be checked by hand rotating them with the wheels in the door gap. If the wheels do not rotate freely, the rollers are jammed or twisted. Jammed rollers are usually caused by a buildup of residue, such as powdered graphite, burned off ash from an improper lubricant, or iron oxide. This residue can sometimes be removed by blowing out the cage with compressed air. If this does not free the bearing, or if the bearing has twisted rollers, it must be disassembled, cleaned with solvent, bearing cages straightened, and re-assembled.

Twisted rollers can be caused by forcing a jammed wheel to turn. Twisted rollers can be diagnosed by turning the wheel in both directions. When checking for twisted rollers, the wheel should be given several rotations, because the wheel may rotate freely until it moves against the side thrust washer. If the wheel moves sideways, the rollers are twisted. A wheel with twisted rollers will jam because the wheel will move sideways until it wedges against the side thrust washers. This can gouge the shaft and the liner in the wheel. If the roller assembly has twisted, it must be straightened or replaced.

If a turning wheel makes a rumbling sound, there are flat spots on the shaft, rollers, or wheel liner. The cause of flat spots is operation of the wheels above their design temperature, excessive loading, or both. If flat spots are found, the damaged parts must be replaced, and the cause of the damage corrected.

Wheels must be aligned so that all wheels are:

• At the same elevation so they carry their share of the load.
• At the same sideways location so the flanges are not overloaded at transfer points.
• Vertical.
• Exactly parallel to the direction of travel - no toe in or toe out.

It is imperative when installing a replacement wheel to check its elevation.  Often, old wheels are worn and smaller than new wheels. The new wheel would then carry a severe overload if its elevation is not corrected by reducing the number of shims.

Rails must be aligned so that the furnace rails and the foundation rails are all:

• Level.
• At the same elevation, especially transfer points.
• Parallel.
• Properly spaced.

Rails must be properly supported so they remain level under load.  If the structure supporting the rails deflects under load, it will probably cause overloading of some wheels. Generally, the rail support should be solid steel shimmed tight from the foundation up to the rail.

Sideways support is also important so the rails do not spread and cause the wheels to ride the edge. Wheels riding the edge may cause worn rails and chipped wheels. If tie rods and spacers are used between rails, clearance must be provided for expansion of the rods and spacers.

Rails may become dented where the wheels set, especially in services over 1000°F. A dent of only 1/8" deep can greatly overload pulling equipment. This could be up to a 100% increase in pull required. Dents should be repaired by welding and grinding when the depth reaches 1/16".

The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.