An Introduction to Hydro- mechanical Transmissions

Product Development & Global Technology

An Introduction to Hydro-mechanical Transmissions

Mike Cronin

28 Nov 2012 (Previously titled: “Cost Savings With Hydro-mechanical Transmissions”)


Introduction

• Topics Covered: ▪ Speaker Background ▪ Simple Hydro-mechanical Transmission Schematic ▪ Current Industry Examples ▪ Important Design Characteristics ▪ How Fuel Is Saved ▪ Other Beneficial Characteristics

• Caveats:

▪ Not A Design Guide ▪ Limited To Off-road Perspective

Hydro-mechanical transmission architectures are growing in popularity as a cost effective means to provide continuously variable transmission (CVT) functionality in the heavy-duty off-road market segment. This presentation will introduce the audience to the basics of operation and commonly used terms.


Mike Cronin’s CV • Joined Caterpillar upon graduation from Michigan State University with a

BSME in 1973

• Entire career working on off-road drivetrain performance and design: ▪ Hydro-mechanical transmissions for a broad range of applications ▪ Electric drive for TTT ▪ TTT & Belted machine steering systems and components ▪ Assorted lower powertrain projects ▪ Assorted powershift transmission concepts

• Drivetrain related patents

▪ 23 Granted ▪ 10 Pending ▪ 9 Notifications

• Retired in 2010

• Rejoined Caterpillar’s Drivetrain Research Dept on a part time basis to

continue work on hydro-mechanical drivetrains.


Engine Gear

System Gear

System

Variator

Transfer Gear And/or Axle Reductions

Wheel Or

Sprocket

Transmission

General Parallel Path Transmission Schematic a.k.a. Split Torque, Power Split, Hydro-mechanical, Electro-mechanical etc.

• A variator is a device that can vary the speed or torque ratio across its two shafts in a continuous manner.

• Several types are available: hydraulic, electric, traction etc.

• In addition to gears these systems may contain clutches, brakes or other familiar transmission components connected in various ways.

• A very large number of combinations are possible.

There must be at least one planetary

PlanetaryGear

System

Hydraulic Power Path

Mechanical Power Path


Speed 1

Speed 2

General Planetary Gear System

Planetaries Are Splitters and Adders

Output speed is the sum of the input speeds.

Output Speed = A*Speed1

+

B*Speed2

Speed Adder

Torque Splitter

Output Torque1 = A*Torque

Output Torque2 = B*Torque

Torque


Engine Final Drive

Wheels or

Tracks

Pump Motor

Hydro-Mechanical CVT

Engine Speed

General Planetary Gear System

Variable Speed

Output Speed =

A*Engine Speed +

B*Variable Speed

Even if engine speed is constant, if the other speed is variable then the output

speed is still variable.

Hydrostatic Variator Shown


Why choose hydro-mechanical?

• Scaling ▪ Variators are only available in a limited number of sizes. ▪ A given size variator matches to a larger machine when used in a hydro-

mechanical configuration. ▪ Larger machines now have access to CVT behavior.

• Efficiency

▪ The power path carries a fraction of engine power ▪ Less hydraulic power means fewer losses.

– Less fuel – More power to ground

• Cost

▪ Most cost effective CVT technology for 200-400 hp wheel loaders.


Hydro-mechanical Transmission Examples AG

IVT Eccom/IVT/S-matic Vario

IVT CVT


Hydro-mechanical Transmission Examples Wheel Loader

Cat CVT

Dana/Rexroth HVT

ZF cPower


Important Design Differentiators

• Variator Type ▪ Discussed Above

• Coupling Type

▪ Input ▪ Output ▪ Compound Split

• Number of Ranges or Modes

▪ One ▪ Two ▪ Three ▪ Four


▪ Variable Pump ▪ Fixed Motor ▪ Integrated, Inline Package

▪ Used in ZF Designed cPower Shown @ 2010 Bauma ▪ Variable Pump ▪ Variable Motor ▪ Pump and Motor Displacements Linked ▪ Motor Displacement Decreases while Pump

Displacement Increases ▪ Integrated, U Package

Variator Examples


Input Coupled / Output Split

In Out

Planetary Gear

System

Double Coupled / Unsplit

In Out

Planetary Gear

System

Output Coupled / Input Split

In Out

Planetary Gear

System

Uncoupled / Compound Split

In Out

Planetary Gear

System

Explaining the term “Coupled”

The Variator is Coupled to the Output The Variator is Coupled to the Input

Cat CVT JD 6000, 7000 IVT

S-Matic Graziano Fiat/Ag

ZF Eccom Rexroth HVT 2nd,3rd

914 ST125 1st

Rexroth HVT 1st

D6E D7E

795AC

Vario 8000 IVT 1st,3rd

ZF CP

Prius Dual Mode/AHSII/EP40 1st

8000 IVT 2nd,4th

ZF CP 2nd,3rd

Dual Mode/AHSII/EP40 2nd

Hyd

raul

ic

Varia

tors

El

ectr

ic

Varia

tors

Volt


Engine Speed vs Ground Speedw Discrete Step Transmission

600800

100012001400

16001800

20002200

0 5 10 15 20 25Ground Speed mph

Engi

ne S

peed

rpm

Discrete Ratio Powertrain Schematic

Engine Transmission

Aux Loads

• Discrete Ratio Transmission – Several selectable gear ratios – Slipping clutch for launch


Wheel Or

Sprocket

Ground Speed 2

One Engine Speed Possible at

Ground Speed 2

Ground Speed 1

Two Engine Speeds Possible at Ground

Speed 1


1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

Engine Speed rpm

Engi

ne G

ross

Pow

er k

W

Hypothetical Engine Fuel Rate Contoursg/min

800-900

700-800

600-700

500-600

400-500

300-400

200-300

100-200

0-100

Approximate Minimum Fuel Consumption

Ground Speed 1 Ground Speed 2


Continuously Variable Ratio Powertrain Schematic

Engine Transmission

Aux Loads

• Continuously Variable Transmission – Ratio continuously managed for

performance and launch


Wheel Or

Sprocket

Engine Speed vs Ground Speed

600

8001000

12001400

1600

18002000

2200

0 5 10 15 20 25Ground Speed mph

Engi

ne S

peed

rpm

Functional Definition of CVT: Engine speed can be placed and maintained at the desired level regardless of ground speed.

Any Engine Speed Possible at Any Ground Speed


END

An Introduction to Hydro- mechanical Transmissions

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