Within the crane industry, there has been considerable confusion as to the definitions of CMAA C and D services, as well as HMI ratings of H3 and H4. In the past, Class C has been regarded as packaged hoist equipment, while Class D has been regarded as open deck machinery. As packaged hoist manufacturers increased the duty ratings of their hoists from H3 to H4, packaged hoists then began to be used for Class D service. In some cases, this has resulted in premature failures of Class D packaged hoists when operating in a Class D rating.
Crane Manufacturers Association Specification No. 70
defines Class C and D service as:
Class C (Moderate Service)
This service covers cranes which may be used in machine shops or papermill machine rooms, etc., where service requirements are moderate. In this type of service, the crane will handle loads which average 50% of the rated capacity with 5 to 10 lifts per hour, averaging 15 feet, not over 50% of the lift at rated capacity
Class D (Heavy Service) This service covers cranes which may be used in heavy machine shops, foundries, fabricating plants, steel warehouses, container yards, limber mills, etc., and standard duty bucket and magnet operations where heavy duty production is required. In this type of service, loads approaching 50% of the rated capacity will be handled constantly during the working period. High speeds are desirable for this type of service with 10 to 20 lifts per hour averaging 15 feet, not over 65 percent of the lists at rated capacity
Hoist Manufacturers Institute Specifications
define Class H3 and H4 service as:
General machine shop, fabricating, assembly, storage, and warehousing. Loads and utilization randomly distributed. Total running time not over 25% of the work period.
High Volume handling of heavy loads, frequently near rated load in steel warehousing, machine and fabricating shops, mills, and foundries, with a total running time not over 50% of the work period.
ACECO has found that the following applications generally require Class C service cranes.
CMAA Class C/HMI H3 Service Applications
Papermill Wet End
For Class D, the heavy service class, we have found the following applications require the Class D service.
CMAA Class D/HMI H4 Service Applications
Structural Steel Fabrication
Papermill Dry End
What do the C/H3 and D/H4 ratings mean in practical terms?
ACECO differentiates between Class C and Class D in the choice of wheels for top running cranes and hoists for Class C, ACECO uses the gear rim type wheels. This is an economical design choice and has demonstrated years of satisfactory service in the Class C applications. For Class D service, ACECO specifies rotating axle type wheel and bearing systems. This system allows for longer bearing life.
Changes a customer can expect to see on a crane when ordering a D, instead of a C crane includes a slight increase in girder section, as a result of the weld allowable stress range. As the class of service increases, the allowable stress range decreases, which may necessitate an increase in girder section size to reduce stresses. Please note this is a cyclic design issue as the allowable static stress is the same for both Class C and D service. Several mechanical items also increase in size or rating when moving to Class D. Sheaves and drums both increase in size. Class C sheaves and drums are required to be 18 times the rope diameter, whereas in Class D it is required to be 20 times rope diameter. This increase in diameter reduces rope stresses and thereby increases rope life. Changes also occur in the gearing design to produce gears of greater durability for Class D service. The major change in the mechanical is the increase in L-10 bearing life from 5000 hours at Class C service to 10,000 hours in Class D service. In a Class D application, the electrical design also may change. A Class D application may require increasing the contactor size for the motor controller based on its rating (horsepower).
The HST design standard requires a 20% of ultimate strength design similar to the CMAA design standard. However, the HST has an exception requiring 35% of ultimate strength design where deformation of the material can occur. Diameter requirements under the HST standard do not increase with increasing duty cycle. In the HST, the minimum drum diameter requirement is 16 times rope diameter. Note that this is a smaller diameter than either Class C or D service. The drum diameter in the HST standard is 16 times the rope diameter, which would be equivalent to CMAA Class C service. Bearing life for H3 and H4 is similar to CMAA Class C and CMAA Class D, respectfully.
It should be understood that the above are general guidelines. When questions arise, please consult American Crane’s Engineering Department for a complete duty cycle sizing analysis to assure the hoist and crane selected performs adequately for service.