Two key factors influence the “toughness” of overhead crane wheels:
- The hardness of the raw material used to make the wheels—such as alloys, low-carbon steel or medium-carbon steel.
- The heat treating techniques used to enhance the metal’s hardness.
Medium-carbon steel is the most commonly employed material in the crane wheel industry due to its wide availability and lower cost.
As a general rule, the greater the carbon content of steel, the greater the hardness potential. Thanks to advances in heat treating technology, the hardness of crane wheels made with medium-carbon steel has been increasing. This toughness counts most at the crane wheel tread; location where the tread meets the rail. A hardened crane wheel tread reduces wear and extends the life of crane wheels. Note, if the wheel hardness exceeds 65 Rockwell “C” the wheel might be “too hard.” The wheel becomes brittle and has the potential to crack if it comes in contact with part of the rail that has missing pieces or other sharp objects.
On the other hand, top of the wheel flange and the outer part of flange should not be hardened. Flanges require ductility so that they can bend, and not break, when subjected to lateral forces, such as a misaligned rail. You don’t want broken wheel flange pieces endangering your employees. Inside of the wheel flange is to maintain the same depth and hardness as the tread to maximize the wheel life!
While many manufacturers can heat treat and harden crane wheels and other steel crane parts, results vary from one vendor to another. When purchasing crane wheels, you want to make certain that you acquire them from a company with the expertise to provide consistently deep hardening of the crane wheel tread. Manufacturer should have hardness recorded upon shipment of the wheels.
Determining if the surface hardness of crane wheel tread is a critical aspect of wheel hardness, and relatively simple to measure. More difficult to measure, however, is the wheel’s depth of hardness. Because this measurement can be difficult to determine, it’s often neglected, leading to potential problems. When hardening doesn’t go deep into the wheel tread, the crane wheel is subject to spalling—that is, metal fragments breaking off the surface, shortening a wheel’s life.
But a wheel’s lifespan depends on more than hardness alone.
The wheels, the only parts of the crane that make direct contact with the runway, can be the weakest link of the crane. Commonly, they are the first crane parts to show the effects of crane problems, such as misaligned rails or un-squared crane.
Periodic inspection of cranes, including the wheels, will detect premature wheel wear and provide clues to correct problems before they can cause substantial damage.
I recommend measuring and documenting the thickness of wheel flanges so you can keep track of the rate of wear between inspections. If wheels take 10 years to wear a quarter of an inch, you may have little cause for concern. But if this happens in a few months, you have a serious problem.
For more information on crane wheels, contact a Konecranes representative today.