Light Duty, Medium Duty, Heavy Duty
What's the difference...

Personal reflections by: Ulf Henricson Integrated Drives Sweden AB.
The material is intended to give a general understanding of hydraulic drive systems of the 21th century and assist engineers that experience limits in traditional transmission technique.

First of all:
The following is not the truth. It is my personal reflections based on 25 years in close contacts
with the world wide hydraulic industry.

All machines are designed for a certain duty.
1) Examples Combustion engines:

a) Two stroke gas engines are normally designed for use in light duty applications
as in light motor bikes, chain saws, etc.

b) Four stroke gas engines are normally designed for use in medium duty applications
as in personal cars, heavy motor bikes etc.

c) Diesel engines are normally designed for use in heavy duty applications as in trucks,
construction mobile machines, marine propulsion etc.

It is accepted that a diesel engine is not fit to light motor bikes.
A two stroke gas engines is not installed in big trucks.

This examples are understod by all technicians around the globe.

2) Other well known technique:

a) Electric motors are normally designed for a rated output power.
If other working conditions than rated, the motor output power has to be
rated based on actual driving conditions.

<See Manufacturer technical material>

b) Industrial gear reducers are normally designed for Light Duty conditions.
For Medium Duty or Heavy Duty applications the input power has to be reduced.
The derating factors are based on shock loads coming from the driven machine,
frequent starts and stops, driving and braking in both directions etc.
A commonly used derating system is based on the well known
AGMA - factor. (American Gear Manufacturers Association)

<See Manufacturer technical material>

All electrical engineers has been educated in the design of electric motors.
Rating of output power is known by all of them.
All mechanical engineers have been educated in gear box technique.
The knowledge in dimensioning of gear boxes for different duty is therefore also well known.

It's established technique.

When it comes to hydraulic technique the basics are different and more complex.
3) Hydraulic Cylinders:

Almost all Hydraulic Cylinder manufacturers supply cylinders for Light Duty, Medium Duty and Heavy Duty applications.
The differences are accepted and nobody question it.

Light Duty	Medium Duty	Heavy Duty

- The basic design is different
- The materials used are different
- The amount of material used is different.
- The type of seals are different
- The life and reliability is different
- The price is different and accepted.

Nobody applies Light Duty cylinders in steel mills for handling billets in continuos
24 hours / day applications.

Cylinder technique is an established technique.

4) Hydraulic Pumps:

All major Hydraulic Component manufacturers supply pumps for Light Duty, Medium Duty and Heavy Duty applications.

a) Light Duty pumps can be gear pumps.

c) Heavy Duty pumps can be Piston Pumps.

The differences are obvious when one examines this three typical pumps on drawings or dismantled,
but unfortunately there is often a lack of knowledge as to basic difference. Some points:
- The fundamental design is different
- The materials used are different
- The amount of material used is different for the same flow capacity.
- The efficiency is different.
- The life and reliability is different
- The price is different but not always accepted..

It is a true fact that one can see Light Duty gear pumps supplying oil to Heavy Duty cylinders handling steel billets in continuos 24 hours / day applications.
Such hydraulic applications is a miss match of components and cause problems for the end users for obvious reasons..

The pump technique itself is an establish technique, but the technique in utilisation of pumps is not always understod.

5) Hydraulic motors:

First of all:
The following describe Low Speed High Torque Gearless Drives.
In the case of a combination of high speed hydraulic motors with gear reducers, the gear box usually determine the duty factor. (Ref. to AGMA - factors.)

Secondly:
No major manufacturers of hydraulic components do manufacture Light Duty, Medium Duty and Heavy Duty drive units. In most of the cases they produce Light Duty units only. Most likely because of the similarities between physical sizes of the products, machines used in the manufacturing process, basic technique involved, subcontractors used, logistics, sales and service organisation, customer base etc. Some large hydraulic companies have co-operations with manufacturers of Medium Duty LSHT - units or Planetary Gear box manufacturer.
Non of them are today manufacturing big Heavy Duty LSHT - drive units.

Thirdly:
The Low Speed High Torque drive unit manufacturers do not see themselves as hydraulic companies. They use the basics in the hydraulic technique to generate large rotating forces. As a comparison, cylinder manufacturers use hydraulic technique to generate large linear forces.

Forthly:
The hydrostatic way of generating force or torque becomes more cost efficient than traditional mechanical transmission systems when big torque or force are needed.

6) Examples of Light Duty, Medium Duty and Heavy Duty drive units.

a) Gerotor or Orbit motors for light duty applications. Danfoss, TRW, etc.

b) Excenter motors for medium duty applications. SAI, Calzoni etc.

c) Cam Ring motors for Heavy Duty applications. Hägglunds, Flender, etc

The differences are obvious when one examines this three typical motors on drawings or dismantled,
but unfortunately there is often a lack of knowledge as to basic difference. Some points:

- The fundamental principals are different
- The psychical sizes are different for the same torque capacity.
- The life and reliability is different
- The efficiency is different.
- The price is different but not always accepted.

The sad fact is that one can see light duty gear pumps supplying oil to Medium - or even Heavy Duty LSHT drive systems in continuos 24 hours / day applications. Such applications are a miss match of components and cause the end user problems for obvious reasons.

The motor technique itself is an establish technique, but the technique in applications of motors is not always understod.

7) Conclusions:

The true facts is that the hydraulic technique is very reliable.

a) Hydraulic systems are often used in applications where other transmissions is not feasible.
b) Hydraulic systems often operates in harsh environmental conditions.
c) Hydraulic systems often operates with contaminated oil.
d) Hydraulic systems often has overheated oil.
e) Hydraulic systems are often serviced only when exchange of failed components
f) Etc.

Hydraulic systems with such abuses listed above often continue to operate even if reliability is compromised.
One can wonder what should happen if electric systems should be miss matched and abused as much as hydraulic systems. Most likely it should be very dangerous to work in the industry, in offices or switch on the TV at home.

Fortunately one can see a change in the attitude towards hydraulics:
- End Users have appreciated the benefit hydraulics offer with regards to productivity, controllability, simplicity, space savings etc.

- End Users have also noted that hydraulics are trouble free with high reliability and low maintained costs when built and installed by professional people.

- End Users now build up internal hydraulic competence.

- End Users endeavour to have one company responsibly for the entire installed hydraulic system. This makes it difficult for non competent companies to supply.

- OEM's realise that a well designed hydraulic drive system often gives a welcome competitive edge.

As general rules to users of hydraulics the following guidelines when comparing different systems is useful:

1) Compare apple with apple.
- The same Duty Cycle.
- The same AGMA factor.
- The same environmental factors.

2) Combine right components.
- Matched components.
- Do not forget piping.

3) To keep in mind:
- Power and speed is cheap
- Torque and life is expensive.

8) Examples / Analogies:

The analogy with big Marine Diesel engines and The Hercules Concept are as follows:

# In principal no torque limits
# No gear boxes
# Tailor made of standard modules
# Simple, rough and reliable mechanics
# Sophisticated controls
# Costs effective

Examples of suitable applications for The Hercules Concept:

The Hercules Concept in Sugar Industries

The Hercules Concept in Mining

The Hercules Concept in Mineral processing

The Hercules Concept in Marine and Offshore

The Hercules Concept in Bucket Wheels

The Hercules Concept in Thickeners/Clarifiers