Pressure and Speed Limits
Transmission Design Elements
Sauer-Danfoss
Hydraulic
Unit Design
Fluid
Quality
Pressure
and Speed
Ratings
Transmission
Design
Component
Sizing
Circuit
Design
Customer
Machine
Design
Pressure &
Speed Limits
BULLETIN PURPOSE
• Provide understanding
of ratings and their
relationship to expected
life.
BLN-9884
Key Concepts
• Maximum and Rated pressure
• Expected life
• Maximum and Rated speed
• Duty cycle
Low
PRESSURE
Hi
Load Vs Life
Short
LIFE
Long
Unit Life
• Upper life limit
– Rotating group
– Bearings
• Actual life
– Heat
– Contamination
– Viscosity and lubricity of fluid
Max vs Rated Pressure
PRESSURE
MAXIMUM
CONTINUOUS
TIME
Pressure
• Maximum Pressure
– Highest intermittent pressure allowed
– Determined by max. machine load demand
– Less than 2% of total operating time
• Continuous Pressure
– Pressure expected when performing normal function
– Normal input power, max. pump displacement
– Weighted average
Speed Limits
Consider Operating Conditions
• Normal driving
• High idle/no load
• Overrunning load
– Must know something about the frictional
characteristics of engine
Speed Ratings
Swashplate Units
If Above
Continuous,
Contact Sauer- Bent Axis (Series 51)
Danfoss
Not always block lift first
Rated Speed
• Life inversely proportional to speed
• Highest speed recommended at full power
• Highest speed for normal life
• Varies with angle
• Must always check Maximum speed limit
Maximum Speed
•
•
•
•
•
Highest operating speed permitted
Risk immediate failure if exceeded
May loose driveline power if exceeded
Highest negative power (downhill braking)
Catalog ratings…:
– pumps: always reflect maximum swashplate angle
– fixed motors: always reflect maximum swashplate
angle
– variable motors: both maximum and minimum angle
are published
Cavitation damage
Over speed
Over speed
Design Speed Considerations
•
•
•
•
•
•
•
Full power
Full braking
Prime mover speed change
Pump drive ratios
Tolerance and drift of engine governors
Hydraulic unit volumetric losses
Tolerance in hydraulic unit displacement
Maximum Speed Considerations
• Maximum motor operating speeds must be
determined and compared to Maximum speed
limits.
• For vehicle propel drives this usually occurs
during down-hill braking conditions.
• Another severe condition is zero delta pressure
at high engine idle speed.
Drive Design Finalization
• Operating parameter check
• Pressures < recommended limit
• Speeds < recommended limit
• Expected life achieved
Secondary Braking
Insist that your customers provide a
secondary means of braking
on ALL vehicle propulsion systems!
Check Speed Limits
• Check max angle limits of base
units in catalog
• Check charge pump limits
• Check reduced displacement
variable motor limits
Reduced Angle Speed Calculation
• Swashplate Units:
Speed at

Tan Max Angle
 

 Speed at
= 

 reduced angle  maximum angle Tan Reduced Angle
• Bent Axis Units:
Speed at
Speed at
Sin Max Angle 
 
 



=
 

reduced angle maximum angle Sin Reduced Angle
Do not use speeds greater than the "Reduced
Angle" speeds shown in the tables!
Series 51 Speed Limits
• Maximum Speed is the highest speed permitted.
Rated Speed
rpm
Maximum Speed
rpm
Frame
Size
(Full Displ.)
(Min. Displ.)
(Full Displ.)
(Min. Displ.)
080
3100
5000
4000
6250
110
2800
4500
3600
5600
160
2500
4000
3200
5000
250
2200
3400
2700
4250
Reduced Angle Motor Example
• What is Rated speed limit of 51V110 at 22 deg
speed @ 22 deg
= speed @ 32 deg * (sin 32)
(sin 22)
= 2800
*
(sin 32)
(sin 22)
= 3960 rpm
• catalog reduced angle displacement max = 4500 rpm
• use lesser of calculated vs. catalog, i.e. 3960 rpm
The Million Dollar Question
D
T
U
Y
C
C
Y
L
E
Duty Cycle Definition
• What is a Duty Cycle?
DUTY CYCLE 1
Condition
% Time
1
2
3
4
5
6
7
8
1.0%
2.0%
5.0%
16.0%
30.0%
34.0%
9.0%
3.0%
SUM:
100.0%
Delta
Pressure
Speed
(bar)
(rpm)
450
350
250
200
180
150
100
50
1700
1750
1840
1900
1950
2000
2100
2200
Cumulative Damage
• Cumulative damage, or more precisely, cumulative
fatigue damage, is the total damage to a component
as a result of repeated cyclic loading.
– Stress levels are not great enough to cause the component
to fail under static loading.
– Related to crack initiation within the material.
Duty Cycle Calculations
• Calculation procedures vary.
• Procedure has dramatic effect on predictions.
• Sauer-Danfoss uses conservative procedures.
• Competition: Compare calculation procedures.
What is Weighted Pressure?
• Weighted pressure serves as a general indication of
a duty cycle’s severity
– The greater the weighted pressure, the lower the predicted
life.
– Of the three criteria for weighted pressure, this one is the
least demanding.
Calculating Weighted Pressure
• Weighted pressure equation:

Pw = %Time1 P1
exp
+ %Time2 P2
exp
+ ... %Time j Pj
exp

1 exp
• Equation does not include any information
concerning the shaft speed at each condition.
• For duty cycles with equal weighted pressures, the
life will be the same only if the speed at every
condition is the same for both duty cycles.
Weighted Speed
• Every duty cycle also has a weighted speed:
Weighted Pressure = Root Mean Octic Pressure = (SUM(Column G))^(1/ EXP)
Weighted Speed = SUM(Column H)/ (Weighted Pressure)^EXP
Predicted L20 Kit Life Using Load-Life Equation & Weighted Values
263
2206
4875
bar
rpm
hours
Predicted L20 Kit Life Using %Time/Life Method
4875
hours
Weighted Speed
• Equation for weighted speed is
RPMw =
%Time1 RPM1  P1 
exp
+ %Time 2 RPM2  P2 
exp
 ...%Time j RPM j Pj 
Pw  exp
• Weighted Speed is not calculated using the
traditional “weighted average” formula:
RPMw  %Time1RPM1 + %Time2 RPM2  + ...%Time j RPMj 
exp
Duty Cycle Reduction
• Knowing how to calculate the predicted life or the
weighted pressure for a customer’s duty cycle is
good thing to know, but…
• How do I measure a duty cycle?
– Usually too many conditions
– Can’t test everything at once
Hi
Typical Pressure Distribution
MAX LOAD
PRESSURE
HILL CLIMB
NORMAL POWER
LEVEL GROUND
Lo
IDLE
% Time
Combining duty cycles
Max load
2%
Hill climbing
20%
Normal power
40%
25%
Level ground
Idle
13%
Life estimate
Thank You
• Questions?