Analysis and Elimination of Oil Leakage in Crane Horizontal Hydraulic Cylinders

The horizontal hydraulic cylinder is the actuator that completes the horizontal extension and retraction of the outriggers before and after the truck crane performs lifting operations. During use, due to its relatively small load capacity and infrequent movement, oil leakage is not easily noticed.

1. Determination of the oil leakage site

There are generally three types of external leakage in hydraulic cylinders: the first is oil leakage along the inner seal between the piston rod and the guide sleeve; The second is the oil leakage along the outer seal between the cylinder barrel and the guide sleeve. The third reason is the oil leakage caused by defects such as casting pores, sand holes and shrinkage porosity in the cast guide sleeve.

When disassembling and inspecting the horizontal hydraulic cylinders of the QY8B and QY8E truck cranes, it was found that the oil leakage situation was almost always the same as the second type of oil leakage phenomenon mentioned above.

Analysis of the causes of oil leakage

The seal between the cylinder barrel and the guide sleeve is a static seal. Possible causes of oil leakage include: Poor quality of the sealing ring: Insufficient compression of the sealing ring; The sealing ring is scratched or damaged. The quality of the cylinder barrel and the surface processing of the sealing groove of the guide sleeve are rough.

When disassembling and inspecting the horizontal hydraulic cylinder with external leakage, it was found that some O-rings 4 had been scratched. This was due to the fact that the outer sealing O-rings had undergone an unsmooth chamfer during the assembly of the guide sleeve.

When assembling, first fill the inner key ring groove of the cylinder barrel with a rubber gasket of appropriate thickness or a special filling ring (to prevent scratching the outer sealing ring of the piston and the guide sleeve), then install the guide sleeve with the sealing ring and the piston rod from the right end of the cylinder barrel. At this point, remove the rubber gasket or the special filling ring, insert the inner key ring, and then use oil pressure to push the guide sleeve out until it is blocked by the inner key ring After installing the retaining ring and the spring retaining ring, it is in the state shown in Figure 1. During this process, when the O-ring outside the guide sleeve passes over the non-smooth chamfer A at the oil port recess of the cylinder barrel (hereinafter referred to as the non-smooth chamfer A), it is possible to be scratched by it. There are two situations where A non-smooth chamfer is passed over: If L1<L2 and the O-ring passes over a non-smooth chamfer A, it is due to improper assembly. After checking the original design drawings, it is known that L1=31mm, L2=39mm, and L1<L2. Therefore, the design is poor.

In addition, during disassembly and inspection, it was found that the chamfer of the key ring groove inside the cylinder should be 2×15o, but it was often overlooked during processing and turned into 6×10o. After assembly and positioning, the inner key ring groove was very close to the O-ring sealing groove (only 5mm), and the out-of-tolerance chamfer caused the two grooves to be connected, resulting in damage to the O-ring when it was squeezed, causing oil leakage in the gap.

3. Methods for eliminating oil leakage

In terms of design, it should be ensured that the relationship between the cylinder barrel size L1 and the guide sleeve size L2 is L1<L2. After improvement, L1=39mm and L2=35mm, as shown in Figure 3.

(2) Before assembly or after processing, the non-smooth chamfer A of the cylinder barrel and the chamfer of the inner key ring groove should be smoothed with fine sandpaper or oilstone.

(3) During assembly, the position of the guide sleeve should be controlled to ensure that the O-ring does not pass over the rough chamfer A.

When processing the cylinder barrel, the surface roughness of the dimensions shown in Figure 3a must be strictly controlled. In conclusion, to eliminate the oil leakage between the sealing ring at the cylinder barrel and the guide sleeve, guarantees should be provided from three aspects: design, processing and assembly. That is, it is necessary to prevent the sealing ring from being scratched during assembly or having insufficient compression after positioning. At the same time, the quality of the sealing ring and the surface processing quality of the sealing grooves of the cylinder barrel and guide sleeve should all be controlled as required.