ECO-FRIENDLY LINEAR MODULE THROUGH HYDROSTATIC
BEARING CONFIGURATION
Introduction
With the rapid progress electronic and optical devices, ultra-precision and
micro-machining technologies are increasingly required to meet high
precision. Linear motion axes are ever-present in manufacturing systems and
therefore in order for ultra-precision machines to meet the high accuracy
demands, ultra-precision linear motion axes would need to be developed.
Linear and angular bearing technology play an important role in linear
motion axes and ultimately determine the machine tools performance. It is
well known that friction has a major influence on the positional accuracy.
This is due to its high non-linearity, and friction degrades the dynamic
performance and positioning accuracy .
Objective
In order to eliminate friction from the linear module, a linear motor will be
used as the drive and as the guiding element, hydrostatic bearings will be
used.
Hydraulic Power Pack Design
The objective of this project is to design and manufacture a hydrostatic module suitable for micro-machining
purposes. The module will consist of a hydrostatic guideway with a suitable hydraulic power pack. The characteristics
of the module will have to be evaluated, firstly with a common hydraulic fluid and then with a more environmentally
friendly fluid.
The module will be used in the development of a micro-lathe and a micro-EDM machines in the future.
Hydraulic Circuit
6
7
#
5
Hydrostatic Guideway Design and Concept
2
Guideway Elements
3
P
M
LINEAR MOTOR :
FORCER
4
GUIDE
PAYLOAD:
SPINDLE /
WORKPIECE /
TOOL HOLDER
RAIL
8
RAIL
Function
Actual Part
1
Sealed tank
Glass Jar
2
Hydraulic pump
12V Bosch fuel injector
pump
3
Relief valve
Bosch fuel pressure
relief valve
4
Variable resistor with a
ball valve
FESTO pneumatic
variable resistor
5
Pressure gauge
Bourdon-tube gauge
6
Inlet restrictors
M6 set screw
7
Bearing pad resistance
Bearing pad
8
Sump
Guideway base
1
8
PADS
LINEAR MOTOR :
MAGNETIC TRACK
PADS
BASE
5
1
1
3
2
#
Guideway Concept
2
5
Part
4
1
Base
2
Guide
3
Rail
4
Manifold
5
End Plate
Experimental Procedure & Setup
Vertical displacement measurement of Bearings A, B C and D
BEARING A
BEARING C
Proximity Sensor
1
2
Test Rig
Parallel Block
PAD
A deflection of the guide will
result in a deflection of the
parallel block. This deflection
will be observed by a
proximity sensor.
Pressurized fluid inlet
from hydraulic power
pack
O-RING
BEARING B
BEARING D
RESTRICTOR
INLET
PASSAGE
RAIL
3
Experimental Results
GUIDE
22
Fluid flows through inlet passage to the
bearing pad.
33
Fluid flows outwards from the bearing pad
land into the sump.
4
BASE
25
The “used” fluid is collected from the
sump and extracted via the return lines
back to the tank.
44
Return of fluid
back to tank
25
Conditions:
Conditions:
- Guide position = 0mm
- Pay Load = 0 kg
20
Film Thickness [µm]
11
Graph 5 : Baby Oil Film Thickness of Bearings A, B, C and
D vs Supply Pressure
15
Bearing A
Bearing C
10
Bearing D
Bearing B
0
1
2
3
4
5
Bearing D
Bearing B
0
6
1
0
10
20
Measurement of
Bearing A
Stroke : 50 mm
Cycles per test : 2
1 Cycle = 2*Stroke
Time per cycle : 6 s
50 mm
maximum deflection over stroke, δmax [µm]
0
30
25
20
5
6
60
Conditions:
50
40
1 - 2 bar Supply Pressure, with 4kg Load
3 - 5 bar Supply Pressure, with 4kg Load
4 - 5 bar Supply Pressure, with 7.035kg Load
15
ASW 32
30
ASW 32
10
Baby Oil
20
Baby
Oil
5
10
0
-0,05
4
2 - 2 bar Supply Pressure, with 7.035kg Load
0
30
3
Graph 7 : Comparision of the Stiffness of Bearing A (in
N/µm) under various conditions of AWS 32 Hydraulic Oil
and Baby Oil
35
Graph 1: Bearing Gap Deflection vs Guide Position
2
Supply Pressure , Ps [bar]
Graph 6 : Decrease of Maximum Deflection of Bearing A
Over Stroke vs Supply Pressure for AWS 32 and Baby Oil
-10
Bearing C
10
Supply Pressure , Ps [bar]
Force Analysis
-20
Bearing A
0
0
-30
15
5
5
Theoretical Performance Analysis
- Guide position = 0mm
- Pay Load = 0 kg
20
Film Thickness [µm]
High pressure fluid is distributed by
manifold to each bearing pad inlet
passage.
Graph 4: AWS 32 Film Thickness of Bearings A, B, C and D
vs Supply Pressure
1
2
3
4
Supply Presure, Ps [bar]
5
6
0
1
2
3
4
Bearing Gap Deflection [µm]
-0,1
Deflection Analysis
Conclusions
-0,15
Bearing A
-0,2
Bearing B
-0,25
Bearing C
Bearing D
-0,3
Conditions:
-0,35
Stiffness Analysis
- Supply Pressure = 5 bar
- Pay Load = 0 kg
- Initial Bearing Gap = 10µm
-0,4
-0,45
Guide position [mm]
Graph 2: Maximum Bearing Gap Deflection of Bearing A
over Stroke v.s Supply Pressure
Experimental trends
35
Conditions:
Conditions:
30
- Guide position = 0mm
- Pay Load = 0 kg
- Initial Bearing Gap = 10µm
25
Stiffness, k [N/um]
Maximum deflection over Srtoke, dmax [um]
0,5
0,4
0,3
- Guide position = 0mm
- Supply Pressure= 5 bar
- Initial Bearing Gap = 10µm
20
15
10
0,2
0,1
5
0
0
0
1
2
3
Supply Pressure, Ps [bar]
4
5
Deflection over stroke (flatness)
Due to the nature of this design the bearing gap will change over the stroke of the guideway. It is show that in order
to overcome this disadvantage the supply pressure must be at around 5bar.
Guideway stiffness
From the mathematical model it can be seen that the stiffness of the linear module is influenced by the payload.
However the stiffness increases as the supply pressure increases.
Graph 3: Stiffness of Bearing A vs Supply Pressure
0,7
0,6
Theoretical trends
6
0
1
2
3
Supply Pressure, Ps [bar]
4
5
6
Fluid film thickness
The fluid film thickness for bearings A and C are greater than that of bearings B and D. It is suggested that this can
be attributed to the mismatch between the bearing pad resistance and the restrictor resistance.
Deflection over stroke (flatness)
The deflection over the stroke decreases with the increase in supply pressure. In other words the flatness of the
guide improves as the supply pressure increases.
It can be seen that the hydraulic fluid (AWS32 specification) performs better than Baby oil. This is attributed to the
better damping properties associated with the hydraulic fluid.
Stiffness
The stiffness of the guide improves as the supply pressure increases. This trend applies for both fluids. However the
baby oil has better stiffness results compared to the hydraulic oil.
Author : B. Janse van Rensburg
Supervisor : Associate Professor R. Kuppuswamy
Project number 52
2009