Transcript Chapter 3 液壓油

Chapter 3
液壓油
3-1 液壓油之種類
(a)礦物油基
最常使用
(b)合成油
(c)水乙二醇液
(d)油中水型及水中油型乳化液
(e)高水份流體(HWCF)
3-2 說明
‧礦物油基
廣泛使用
價格便宜
抗火性較差(易燃)
(適用溫度<100℃)
可添加添加劑以提高抗火性抗
腐蝕性、抗磨秏性、抗氧化性
…等等
‧其餘四種基本上抗火性較佳
。合成油 不含水，由燐酸酯及氯化碳化烴合
成；不適高溫
。水乙二醇液
含35~55%水
。水中油型(油中水型)乳化液
含35~40%水
。HWCF
含90%以上水
3-3 各種液壓油性質之比較
‧由上表可知，石油系(即礦物油基)除了耐火性較
差外，其餘之性質均屬最優。
3-4 液壓油最重要之物理性質粘度
‧常用之粘度單位有二：
(a) 動力粘度：
(dynamic viscosity)

(b) 動粘度： 
(kinematic viscosity)

2

1 poise = 0.1 N．S/m
-2
1 CP = 10 poise

1 stoke =1 cm /sec
2
1cst = 1 mm /sec
2
§ Compressibility of hydraulic fluids
F=(P0 + P)．A
P0  P0 + P
V0  V0 – V
P
V = A． = V0
Eoil
2
Where Eoil : bulk modulus (unit = bar or N/m )
1
P
Eoil : – V0 (
)T =


等溫下
compressibility coeff.
§ Consider a thin pipe shown as follows
P  D
tangent stress T =
2S
P  D
axial stress A =
4S
P=P1-P2
(a)X2 (長度變化於X2方向)
1. due to T :  T  X 2 where E: Young’s modulus
E A
2. due to A :    X 2 where : Poisson’s number
E
(b) X1(長度變化於X1方向)
1. due to A : A  X 1
T
2. due to T : E
  X 1
E
total strain
X 2
1 T
A
1
εT =

( X 2   X 2)  (T  A)
X2
X2 E
E
E
1
(A   T )
similarly εA =
E
(c) U (圓周長之變化)
P  D
P  D
U = U ． εT =π．D(
– 
)
2S
=
D 2 P
2 SE
(1 
U=πD
U + U = πD(1+ εT)
thus u = πD εT=U εT
(d) D ( 直徑的增加)
2

U
D
P

D=

(1  )

2SE
2
4S

2
)
(e) A (斷面積之變化)
D  D D 3  P
A 
2

4 SE
(1 

2
)
cf.
A
D 2
4
D
dA 
 dD
2
(f)  (管長度變化)

P  D 1
(



)
= ． εA= E
= 2 SE ( 2   )
(g) Vp (體積之變化)
Vp = A．  + ．A
∵V=A．
3
2
= D P (1   )    D (   P  D )(1   ) ∴dV=dA． +A．d 
A
T
4SE
2
4
3
= D P (2    0.5   )
8SE
for  = 0.33
 = 0.25
2SE
2
½[2 -  + 0.5 - ] = 0.917
½[2 -  + 0.5 - ] = 1.0
≒1

D2  
thus Vp =
 P  D
 0  P  D
SE
SE
4
V 0  P
Vtotal  V  VP 
Eoil '
where V : change in volume due to compressibility of oil
Vp : change in volume due to deflection of the
thin pipe
Eoil’: modified bulk modulus
for thin pipe
P
Vtotal= V + VP = V0 Eoil + V0 P ．D
E
1
= V0 P( Eoil +
thus Eoil’ =
1 D
E S
)
1
1
1 D

'
Eoil
E S
Eoil '
1

Eoil D
Eoil
1
E S
Similarly, for thick pipe:
P 2 2 (1  )  3(1  2 )
VP=VP E 
 2 1
Where   d 0 ( outside diam eter)
di
inside diam eter
S
§ Density of hydraulic fluids
  f (T ) T: temperature
Let   1 V (: thermal expansion coefficient)
V T
V= V0．．T
m
V0
m
0


V 0  V 1  T
0 
Ex:

 15C
1   (T  15C )
  0.0007(K ) 1
T  115c
thus T=100℃
V
   T  7%
V0
§ Viscosity of hydraulic fluids
— dynamic viscosity 
— kinematic viscosity  
   f (T )


T : Temperature
T  
For convenience:
  0e   (T T0 )
 : constant  1.8 ~ 3.6 102 / C (for pentroleum based oils )
   f ( P)
P : pressure
P   
for convenience :
  0e kP
k : constant
k  1.7 103 bar1 (for petroleum based oils )
0  viscosity at atmospheric pressure
‧一般液壓系統所使用液壓油之粘度範圍為10~900
cst。更精確之值一般均由製造廠來推薦。
‧若就泵來決定，則可參考下表：
3-5 空穴現象(Cavitation)
‧油中多少都含有空氣。在適當之條件下，油中之空氣會
逸出，造成液體、氣體共同存在管路及元件中。其影響所
及，將產生噪音、振動，甚至可能損壞元件(氣蝕)。
‧空氣可能自油中逸出的二個條件：
(a)壓力降低
(b)溫度升高
‧空穴現象最常發生在泵內(尤其當操作壓力過低時)。一般
可經由泵所發出之異音來判斷是否產生空穴。
Ex: Cavitation of orifice
if P2  Pg
then the dissolved air escapes
(P1  P2 )
v1  0
2v
Def:
coefficient of Cavitation

P2  Pg
(if p2  p g )
1
v2 2
2
From Bernoulli’s
eq.
0
1
1
2
2
P1  v1  P2  v2
2
2

P1  P2 
thus  
v2 2
2
P2  Pg
P1  P2
assume Pg  0
then  
(Pg 一般很小)
P2
P1  P2
P1
1
 1
P2

from experiment   0.4
P1
 3.5
P2
if
P1
 3.5  Cavitation
P2