Physical Phenomena in Hydraulic Transmission
Posted on 05/02/2024
1. Hydraulic impact
1.Cause:
1) The fluid flow is suddenly cut off or reversed causing hydraulic shock.
2) Hydraulic shock is caused by inertial force when the moving working parts are braked or when changing the can.
3) Due to the lagging action of hydraulic components, the system pressure cannot be adjusted in time, causing hydraulic shock.
2. Pressure shock caused by rapid closure of the liquid flow channel. This hydraulic shock is related to the speed of shock wave propagation in the conduit. The faster the shock wave propagates, the greater the impact pressure. At the same time, it also varies depending on the closing speed of the channel. This kind of impact includes two situations, namely complete impact and incomplete impact.
Assume that the time when the channel is closed is t, and the time for the shock wave to reflect from the starting point back to the starting point is T. Then when t<T, it is a complete impact, and all the liquid flow energy is converted into hydraulic energy; when t>T, it is an incomplete impact. Impact, at this time only part of the fluid flow energy is converted into hydraulic energy. The T value is calculated by the following formula
T=2l/a (second)
In the formula: l—shock wave propagation distance (meters)
a—Shock wave propagation speed (m/s)
The propagation velocity a in a pipe filled with hydraulic oil is calculated by the following formula
a＝(E0/ρ)1/2/(1＋E0·d/Eδ)1/2＝a0/(1＋E0·d/Eδ)1/2 (m/s)
In the formula: E0—volume elasticity coefficient of mineral oil (kgf/cm2) is generally taken as E0=1.6×104
E - elastic coefficient of conduit material (kgf/cm2)
δ—catheter wall thickness (cm)
d—catheter diameter (cm)
a0—Sound propagation speed in the conduit (m/s). Mineral oil commonly used in machine tools is generally taken as a0=1320 (m/s).
At the time of complete impact (t<T), the increase value Δp of the conduit liquid pressure is calculated according to the following formula
Δp＝a·ρ·Δv (kgf/cm2)
In the formula: ρ—density of oil (kgf·second2/cm4)
Δv—fluid flow velocity change value (cm/second)
When all liquid flow channels are closed, Δv = v1
When the liquid flow channel is partially closed, Δv=-v2
v1 is the flow rate before the liquid flow channel is closed
v1 is the flow rate after the liquid flow channel is closed
When there is a non-complete impact (t<T), the increase value Δp of the conduit liquid pressure is calculated by the following formula
Δp＝a·ρ·Δv·T/t (kgf/cm2)
It can be seen that the measures to reduce or avoid the hydraulic shock caused by the rapid closing of the channel are:
1) Extend the channel closing time, such as using a pilot valve to slow down the reversing speed of the reversing valve.
2) Reduce the liquid flow speed before the channel is closed, such as opening a buffer groove at the end of the slide valve, etc.
3) Shorten the time of shock wave propagation and reflection, such as shortening the distance of the conduit, or setting up an accumulator closer to the closing part of the channel.
4) Reduce the shock wave propagation speed, such as using a larger conduit diameter and using conduit materials with larger elastic coefficients, such as rubber conduits, etc.
3. Hydraulic impact caused by braking of moving parts
Δp＝ΣmΔv/FΔt (kgf/cm2)
In the formula: m - the mass of the braked moving part (kgf·second2/cm)
F—effective working area of hydraulic cylinder (cm2)
It can be seen from the above formula that in order to reduce the hydraulic impact generated when the moving parts are braked, the time required for braking should be extended or the reduction value of the speed of the moving parts should be reduced. For example, buffer devices such as deceleration and throttling are used at the end of the stroke of the hydraulic cylinder.
2. Cavitation phenomenon
The suction inlet pressure of the hydraulic pump can be calculated by the following formula
ppump=pa-(hλγ+v2γ/2g+·v2γ/2g +pinertia)
In the formula: ppump - hydraulic pump suction inlet pressure suction loss
pa—atmospheric pressure
pinertia—the pressure generated by the accelerated motion of the liquid in the suction pipe and pump chamber
hλ—oil absorption height
v—Flow rate of oil in the suction pipe
Σξ—resistance coefficient of oil suction part
γ—severe
3. Temperature rise
The oil temperature of a general hydraulic system should not exceed 70°C. When calculating heat, the maximum oil temperature is generally 60°C. The oil temperature of the high-pressure system should not exceed 50℃, the oil temperature of the machine tool system should not exceed 25℃ of room temperature, and the oil temperature of precision machine tool hydraulic system should not exceed 10℃~15℃ of room temperature. The properties of each oil temperature range can be found in the table below.
temperature range（℃） | Temperature zone properties | illustrate |
80~100 | Danger temperature zone | Absolutely not allowed to use |
65~80 | critical temperature zone | The life of the oil is shortened and a cooler must be installed. Every time the temperature rises by 8°C, the life of the oil is shortened by half. |
55~65 | Pay attention to the temperature zone | |
47~55 | complete temperature zone | Proper oil temperature |
30~47 | ideal temperature zone | |
20~30 | Normal temperature zone | Dangerous to start, low efficiency |
0~20 | low temperature zone | start danger |
