Spicer Drive Axles Failure Analysis Service Manual - Manual (Page 4)

Failure Prevention
3
Correct Specifications
This section considers three general areas of truck procurement, operation and maintenance that will help prevent axle failure:
• Correct Specifications - matching the axle to the load and expected road conditions
• Drive Practices - training combined with proper use of installed equipment
• Maintenance, Rebuilding and Adjustment - with special emphasis on lubrication
Correctly spec’ing a drive axle for the vehicle and for the job to be done is an essential factor in preventing axle failures. It is ex-
tremely important to spec an axle of sufficient strength to work under the planned vehicle operational environment as well as the
vehicle load rating.
Operating a vehicle outside of the specification (i.e., overloading and/or operating under more demanding conditions) may in-
crease torque requirements and could cause premature damage or failure of axle components.
The drive axle must be designed with strength capable of withstanding the punishment of a loaded truck in operation. All compo-
nents - gears, shafts, bearings, and housing - must meet three essential requirements:
• To carry the load. In most instances, the drive axle supports the major portion of the truck and its payload.
• To withstand the stress of torque developed by the engine and multiplied by the drive train.
• To withstand the stress of impact and shock forces created by road conditions and vehicle operation.
The capability of a drive axle to carry its share of the vehicle load is expressed as axle rated capacity Gross Axle Weight Rating
(GAWK). To prevent axle overloading, the axle rating must be compatible with the weight specification of the vehicle, load, and
expected operating conditions. Overloading will cause damage to the axle assembly parts.
See Spicer Drive Axle Application Guidelines (AXAG-0200).
Torque is Important
The primary function of a drive axle is to provide gear reduction which multiplies torque and transmits it to the driving wheels. In
a truck power train, the engine develops horsepower and delivers the power in the form of torque. The transmission multiplies
this torque and delivers it to the drive axle which multiplies torque a second time.
The drive axle gearing and its related components must be designed to transmit this torque to the driving wheels, so they will
move the combined weight of the vehicle and load over expected road conditions.
Torque requirements vary with different grades and road conditions. Off-highway vehicles such as construction trucks must op-
erate on rough or soft surface roads and steep grades. This requires greater torque for efficient operation. Vehicles with equivalent
load ratings and operating at constant speeds on highways require less torque.
Vehicle Load Ratings
There are two different vehicle load ratings:
• Trucks are rated by Gross Vehicle Weight (GVW) which is the truck weight plus the weight of its load.
• Tractor-trailer rigs are rated by Gross Combination Weight (GCW) which is the weight of the tractor, trailer, and payload.
These ratings, as they relate to engine power and the torque required to move the weight, determine the required axle gearing
strength.
Vehicle Operation
A vehicle is designed to do a certain job under certain conditions. More severe use of the vehicle such as overloading or operating
under adverse road conditions not considered when spec'ing the axle is termed misuse or incorrect operation. Under severe mis-
use, the axle could fail immediately. With lesser misuse, the axle parts could progressively fail over a period of time. When unusual
operating conditions are anticipated, get professional help in spec'ing a drive axle.