Vehicle Drive with Hydraulic Motors - Part 1
FLUID POWER - Design Data Sheet 41
Wheel Motors. Figure 1. Several manufacturers
produce hydraulic motors designed especially for wheel drive of a
vehicle. Since these motors must carry a part of the vehicle weight
as side load on the shaft, the mounting flange is located a short
distance behind the front face. This not only allows the vehicle
weight to be carried more directly over the shaft bearings, but
makes a more compact package, with part of the motor being recessed
within the wheel hub.
Wheel motors, regardless of their HP rating, are designed with a
large cubic inch displacement per revolution. This gives them high
torque output, and matches their speed to vehicle speed,
eliminating gear box reducers.
Because of their direct coupling to the wheel, a separate motor
must be used on each wheel which is to be powered. Several wheels
cannot be driven from the same wheel motor.
Figure 1. Low
speed, high torque wheel motor.
Standard Motors. While any hydraulic motor can
be used for wheel drive, the speed of standard motors is far too
high and their torque is far too low for direct drive to the
wheels. They must be coupled either through a gear box speed
reducer, or through a reduction roller chain drive. However, one
motor can drive several wheels which are mechanically coupled
Maximum vehicle traction is obtained just before the drive
wheel(s) start to spin. When a drive wheel starts to spin, its
traction is reduced, sometimes to almost zero. For optimum vehicle
performance, the hydraulic wheel drive motors should be capable of
producing sufficient torque, under the most adverse operating
condition, to reach wheel spin condition. But torque beyond this
level is of no value.
Traction defines the grip between a powered (or braked) wheel
and the road surface, and is usually expressed as the amount of
horizontal force (drawbar pull), in pounds, that can be exerted
against the road surface without slip. Traction depends not alone
on the motor torque, but also on the materials in the wheel and
road surface, and is directly proportional to the force (vehicle
weight) forcing them into contact. Therefore, in vehicular design
it is important to consider the number of wheels which must be
powered, and the distribution of weight over each wheel.
For simple vehicles running on the Ievel, it may be sufficient
to apply drive power to only one wheel. If possible, this should be
the wheel carrying the most weight. On more sophisticated
applications, involving uphill grades, rapid acceleration, or on
vehicles pulling a trailer, it may be necessary to apply drive
power to two or more wheels to obtain sufficient traction. An
advantage of powering several wheels is that the vehicle is less
likely to become stalled by slick spots on the road surface.
Traction is maximum just before wheel spin.
Multiple Powered Wheels. Additional wheels can
be powered by using a separate hydraulic wheel motor on each wheel
with all of them receiving fluid from a common pump. Wheels or
groups of wheels can be mechanically coupled and driven by a common
pump. In this case, standard motors instead of wheel motors must be
used. Roller chain is often a convenient way of coupling two or
more wheels on the same side of the vehicle if the speed is within
the safe range of roller chain. If wheel separation is so great
that the chain may sag excessively under its own weight, it may be
enclosed in guides or supports.
When linking wheels on opposite sides of a vehicle, a solid axle
is preferred, if practical. A standard rather than a wheel motor
must be used, and it can be coupled through a chain drive on the
If a split axle with differential must be used (for turning
corners), a standard motor can be coupled to the input shaft of the
differential box. However, split axles should not be used unless
necessary, because the vehicle becomes more sensitive to load
distribution. With uneven load distribution the total traction will
be substantially less than if a solid axle were used.
How to Calculate Road Traction
Traction is the adhesive friction of a wheel against the surface
on which it is rolling. The total traction of a vehicle may or may
not be the sum of tractions developed by each individual wheel, as
noted in the following paragraphs.
Traction of each wheel can be calculated by finding the portion
of load weight carried over that wheel, then multiplying times the
coefficient of road traction.
In the U.S. system units, traction expressed in pounds exerted
in a horizontal direction, and must always exceed the calculated
drawbar pull which is required to meet all of the operating
conditions for road surface, grade, acceleration, etc. If it is
not, the wheels will start to spin prematurely, before performance
specifications are met.
Differential Drive. If two powered wheels are
operating a differential, total traction will be that calculated
for the wheel carrying the least weight multiplied times two.
Solid Axle Drive. If two wheels are connected
with a solid axle, traction is computed separately for each wheel
and added together to find total traction.
Dual Wheels. Two wheels, side-by-side on the
same axle give approximately the same traction as one wheel
carrying the same weight. But since two wheels can carry twice the
weight of one wheel, they will produce twice the traction when
carrying twice the weight. The main purpose of dual wheels is not
to get more traction, but to carry more load.
Coefficients of Road Traction
Coefficients of road traction given in the following table are the
same as the coefficients of static friction commonly used in other
engineering calculations. They are, of course, only approximate and
apply to hard, dry surfaces. For other types of wheels and road
surfaces, use the coefficient of static friction for these
materials published in engineering handbooks. Wheels with lugs are
a special case where the coefficient of traction has little to do
with the grip of the wheels on the road. The coefficient for lugged
wheels could be almost anything, depending on the design of the
lugs and the penetration of the road surface.
|Rubber tire on hard surface road
||0.6 to 0.8
|Steel-on-steel (railroad or mine engine)
||0.1 to 0.3
|Track drive, smooth, without lugs
|Track drive, with lugs, estimated
Figure 3. It should be noted that the maximum
traction of any vehicle regardless of the number of wheels which
are powered, is totally dependent on vehicle weight.
Example: A vehicle weighing 5,000 lbs., fully
loaded, and having a coefficient of road traction of 0.8, could
never develop more than 5,000 × 0.8 = 4,000 lbs. traction (drawbar
pull) regardless of how many wheels were powered, nor how high its
engine HP. The only way to get more traction would be to increase
its weight. The advantage to powering several wheels is to take
advantage of vehicle and load weight to get more traction.
In Figure 3, to get maximum traction, all
wheels on a vehicle must be powered, and they must all be
mechanically linked so the one which is most lightly loaded cannot
Figure 3. For
maximum traction, ALL wheels on a railroad or mine engine must be
Figure 4. Cast iron or steel wheels running on
steel rails is a special case of road surface resistance. As the
wheels roll, they slightly indent the rail surface, and must
continually climb out of this indentation. A force, called the
rolling resistance, and expressed in pounds, must be continually
applied as the wheels roll. The amount of this force depends on the
materials in contact, the weight over the wheels, and the wheel
radius, in inches.
For iron or steel wheels on steel rails, the rolling resistance
is about 0.03 lbs. per pound of vehicle weight, and it is inversely
proportional to wheel radius, in inches.
Rolling resistance is largely the result
of indentations in the wheel and in the road surface.
Example: For a 10,000 lb. vehicle with 18-inch
wheel radius, the rolling resistance = 10,000 × 0.03 ÷ 18 = 17 lbs.
This is only the additional horizontal force required to lift the
wheels out of the indentations, and does not include normal road
and starting resistance, bearing friction, and other losses.
*Methods of estimating drawbar pull to meet specifications on road
surface, grade, wind resistance, acceleration, etc., are covered in
"Industrial Fluid Power - Volume 3"
published by Womack Machine Supply Co.
Download a PDF of Fluid
Power Design Data Sheet 41 - Vehicle Drive with Hydraulic Motors -
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this