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A small device of hydraulic cylinder—exhaust device, what are the common problems:
Question 1: Why do we need to set up an exhaust device?
Question 2: What are the common structural forms of exhaust devices?
Question 3: Which hydraulic cylinders are equipped with exhaust devices?
Today, let’s learn about the exhaust device of hydraulic cylinder with the above 3 questions.
1. Why should an exhaust device be installed?
1. Source of air in the hydraulic cylinder
Air will penetrate into the hydraulic system during installation or after a long period of non-operation. Air will also penetrate if the seal is not good. The oil itself always contains 3%-10% dissolved air.
2. Hazards of air in hydraulic cylinders
Accumulation of air causes the hydraulic cylinder to move unsteadily and creep at low speeds. Since gas is highly compressible, it will cause the hydraulic cylinder to creep. When the pressure increases, adiabatic compression will also occur, causing local high temperatures, which may burn the seals. It will cause vibration and noise when starting, and reduce accuracy when switching. Therefore, when designing a hydraulic cylinder, it is necessary to ensure that the gas accumulated in the cylinder is removed in time.
3. Gas removal method
Generally, the air density is smaller than that of oil. An exhaust hole or a special exhaust device can be set at the highest part of the hydraulic cylinder cavity. When the exhaust valve screw is loosened, the oil with air will overflow through the small hole through the conical surface gap. After the gas in the system is exhausted, the screw is tightened to seal the conical surface. The exhaust hole can also be opened at the highest part of the cylinder head, and a long pipe can be used to exhaust the gas to the exhaust valve in the distance.
2: What are the common structural forms of exhaust devices?
Hydraulic cylinder exhaust devices generally have two types: integral exhaust plug and combined exhaust plug.
The integral exhaust plug is shown in Figure 2. It is connected to the cylinder barrel or end face by threads, and the cone surface of the head acts as a seal. When exhausting, loosen the threads, and the air in the cylinder is squeezed out of the gap on the cone surface and discharged out of the cylinder through the inclined hole. This exhaust device is simple and convenient, but the coaxiality requirements of the thread and the cone surface seal are relatively high, otherwise it cannot be sealed after tightening the exhaust plug, causing external leakage.
The combined exhaust plug, as shown in Figure 3 and Figure 4, is generally composed of a screw plug and a cone valve. When the screw plug is loosened, the cone valve is pushed away from the sealing surface by pressure to exhaust air.
1–Cylindrical pin; 2–Valve body; 3–Needle valve
3: Which hydraulic cylinders will be equipped with exhaust devices?
1. GB/T13342-2007 stipulates that hydraulic cylinders should generally be equipped with exhaust devices.
2. GB/T24946-2010 stipulates that digital cylinders can be equipped with exhaust devices as needed.
3. GB/T3766-2015 stipulates that when installing hydraulic cylinders on fixed industrial machinery, they should be able to automatically deflate or provide easily accessible external deflation ports. When installing, the deflation port of the hydraulic cylinder should be in the highest position. When these requirements cannot be met, relevant maintenance and use information should be provided; hydraulic cylinders with inflation chambers should be designed or equipped with exhaust ports to avoid danger. Hydraulic cylinders should be able to exhaust air without danger using exhaust ports.
4. GB/T3766-2001 stipulates that single-acting piston hydraulic cylinders should be equipped with exhaust ports and set at appropriate locations to avoid danger to personnel caused by the spray of discharged oil.
5. GJB638A-1997 stipulates that any place where the mixed air may hinder the normal operation of the hydraulic system should be removed by appropriate methods such as installing an artificial exhaust valve, rather than disconnecting the pipeline or loosening the guide nut.
