Rating Plate Design Categories and Service Class
A below the hook lifting device is any device meant to connect a load to a hoist, and is governed by ASME B30.20. This can include: slings, hooks, spreader bars, rigging hardware, plate clamp and any other lifting attachment. Customers often hire Sparta to design their below the hook lifting devices so over the years I have had the opportunity to design everything from 150ton spreader bars to some funky lifting devices for handling four hundred barrel tanks.
On every B30.20 below the hook lifting device, there is a rating plate that lists the serial number, manufacturer, maximum load, design category, and service class. In this article I discuss the difference between design category and service class.
There are two basic design categories used in B30.20: Design Category A and Design Category B.
Design Category A is used when the magnitude and variation of the lift are known and predictable and when the environmental condition are accurately defined. Design Category B is used when the loads are not predictable and when the environmental conditions can be severe or unpredictable. Category A is really reserved for low cycle lifting equipment used to lift the same product repeatedly in doors. In Alberta, due to the type of work, weather and how hard equipment gets used here, most of what is out there should be Category B.
The second item that you will see on below the hook lifting devices but might not recognize is the service class.
Service class is used by the engineer to communicate the life span of the lifting device. Class 0 equipment equates to a life cycle of up to 20,000 lifts. Class 1 up to 100,000 cycles all the way up to class 4 used when the life span of the device is greater then two million cycles (one million cycles is commonly considered infinite life in the engineering world).
What does the Service Class and Design Class Tell Us?
An engineer uses these two ratings to communicate the type of loading and the lifespan of the product. It is meant as a way of communicating the trade off between durability and robustness with cost.
According to B30.20, the safety factor for Design Category A is 2:1 yielding and for Design Category B is 3:1. So a design Category B lifting device will be proportionally heavier and more expensive than an equivalent Category A device but will also last much longer if used appropriately. This is all governed by fatigue.
Fatigue & Service Life
Fatigue is the engineering term used for describing how material can weaken over repeated use. When you bend a coat hanger back and forth until it brakes – this is fatigue. Fatigue is an interesting concept in engineering as it is where statistics and engineering start to merge. Designing for fatigue forces an engineer to ask the question of what the probable number of cycles, magnitude of each cycle, and whether or not a failure at a given interval will cause catastrophic failure. B30.20 uses the service class and design category to manage fatigue failures. The code states the following:
- 40% of the lifts will be at 50% of the rated load
- 50% of the lifts will be at 75% of the rated load
- 8% will be at 100% of the rated load
- 2% will be at 120% of the rated load
What You Need to Know: for a design category B/service class 0 device (this is the most common lifting apparatus combination), B30.20 is predicting the device only gets lifted at 100% rated capacity 8 out of every 100 lifts.
My question is: are operators aware of this and does it match what actually happens in the field?