Research and Application of the Honing Mechanism of Hydraulic Cylinder Barrel

Research and Application of the Honing Mechanism of Hydraulic Cylinder Barrel

The honing process of the hydraulic cylinder barrel is achieved by applying a certain pressure between the oilstone and the surface of the hydraulic cylinder barrel and achieving a predetermined composite motion between the oilstone and the surface of the hydraulic cylinder barrel with an appropriate cutting amount, in order to machine a hydraulic cylinder barrel surface with high geometric accuracy and good surface quality.

1. Technical requirements for hydraulic cylinder heads

The hydraulic cylinder is the main component of the power slide, and its main technical requirements are shown in Figure 1. The inner hole diameter is 100mm, the length is 2000mm, the material is seamless carbon structural steel pipe ST45, the inner diameter dimension tolerance is H8, and the inner surface roughness is Ra0.4. The straightness is (1-3)/1000. Hydraulic cylinder barrel belongs to precision thin-walled cylinder barrel, with a through-hole diameter of 100mm and a depth of 2000mm. The difficult problem to solve is deep hole machining, and common methods for machining inner holes of cylinder barrels include boring or honing.

The production batch of this part is 8000 pieces per year, which is relatively large. Therefore, a honing machine is designed to process the deep holes of this part, and honing is used to repair the inner holes of the hydraulic cylinder. By using a dedicated honing machine for honing, the productivity is higher than that of precision boring, and higher accuracy can be achieved. Holes processed through honing can ensure the accuracy requirements in Figure 1, and short strokes can also be used to adjust the taper of deep holes in processed parts.

2. Honing principle

Honing processing utilizes the principle of extrusion grinding, and precision machining is carried out on the surface of the hydraulic cylinder by mutual grinding between the grinding tool and the hydraulic cylinder. Fix the parts in the designated position on the workbench with a fixture, and move the workbench to align the parts with the spindle. The honing head is inserted into the inner hole of the hydraulic cylinder, forcing the oilstone strip on the honing head to expand, causing the grinding abrasive to exert extrusion pressure on the machining surface. During the honing process, the honing head is driven by the spindle, rotating while moving, and the surface of the oilstone and the constrained surface of the parts are mutually polished. Due to the harder abrasive particles on the surface of the oilstone compared to the surface of the hydraulic cylinder, it can play a role in honing and cutting the inner surface of the hydraulic cylinder.

Figure.1 Technical Requirements for Hydraulic Cylinder Barrels

The pressure of the oil stone used for honing the inner hole on the hole wall is generally 0.3-0.8 MPa, which can be less than 0.1 MPa for precision honing and above 2-3 MPa for rough honing. Due to extrusion honing, the oil stone and hydraulic cylinder are in arc-shaped contact with a large contact area. The normal pressure of each abrasive particle on the surface of the hydraulic cylinder is only 1/50-1/100 of that during grinding. Moreover, the honing speed is low and cutting fluid is used for cooling and lubrication. Therefore, the temperature in the grinding area can be maintained within the range of 50-150 ℃, which is conducive to reducing residual stress on the machining surface and improving the quality of the grinding surface. Due to the minimal changes in material crystal structure caused by cutting forces, the metal layer on the surface of the part is also less susceptible to damage. The surface quality of the honed parts is very high, and their wear resistance is also good.

In order to flush the chips, avoid clogging the pores on the surface of the oilstone, and reduce the temperature and surface roughness of the grinding area, the cutting fluid needs to be injected into the grinding area at a certain pressure during honing and filtered and reused. In the past, most cutting fluids used kerosene or kerosene with spindle oil. In order to optimize energy consumption, extreme-pressure emulsions are currently being used.

Due to the high stiffness of the honing head in the radial direction, the cutting pressure of the oilstone on areas with smaller inner hole radii can be significantly increased, and there is also more allowance for local grinding. In this way, the surface of the machined part is continuously corrected, gradually approaching the precise cylindrical shape so that the machined inner hole can obtain a very precise geometric shape with minimal errors in ellipticity and cylindricity. The reason why the internal holes of the honed parts can achieve high dimensional accuracy is that honing can precisely adjust and remove the slight allowance. Its error can be controlled within the range of 0.005-0.025mm.

3. Design of vertical honing machine

The vertical honing machine tool consists of a gearbox, control mechanism, spindle and expansion-contraction mechanism, hydraulic transmission system, grinding head and connecting rod, cooling system, bed support components, electrical control device, etc. The overall design is shown in Figure 2.

Figure.2 Vertical Honing Machine

1. Transmission; 2. Control mechanism; 3. Spindle and expansion-contraction mechanism; 4. Hydraulic transmission system; 5. Grinding head and connecting rod; 6. Cooling system; 7. Bed support components; 8. Electrical control devices

The spindle rotation of the machine tool adopts a frequency converter to regulate the motor speed, achieving a step-less speed change of the spindle. The rotational motion of the spindle is transmitted through the internal splines of the gear and the external long splines of the spindle. The gears and bearings in the gearbox are automatically circulated and lubricated by the hydraulic system. The lubrication can be observed from the oil mark on the side of the gearbox.

A feed servo motor drives the reciprocating and introduction motion of the spindle, and the connecting frame connected to the spindle is driven up and down along the guide rod through a ball screw nut mechanism. The reciprocating speed can achieve step-less speed change. The honing head moves back and forth and can be used for long-stroke grinding under normal conditions but can also be changed to short-stroke grinding under special circumstances.

Programmable stroke control can also perform local honing and achieve short stroke control in reciprocating positions. Allow the honing head to perform point or short stroke honing at both ends of the hole during the honing process, and perform honing correction to eliminate the taper of the hole. The spindle feed system driven by a servo motor realizes automatic feed compensation, which can provide high-precision quantitative reciprocating feed with a minimum compensation amount of 0.001mm.

The bed support components consist of columns, bases, horizontal drag plates, workbenches, etc. The column is equipped with a hydraulic system and electrical devices. Select a CNC cross-slide table and operate the right-hand wheel to move the worktable horizontally through a gear and rack mechanism for easy loading and unloading of hydraulic cylinders. The hydraulic cylinder hole can be processed at multiple workstations, and a blocking iron can control the movement position. The left-hand wheel can be operated to move the worktable longitudinally through the screw and nut mechanism in order to align the inner hole of the hydraulic cylinder. The workbench adopts enclosed protection to prevent coolant splashing, open the door and cut off the power, ensuring safe operation.

The hydraulic cylinder honing machine adopts programmable stroke control and processing cycle time control, which can improve honing accuracy and optimize the actual honing effect. Optimize grinding performance and reduce surface roughness of hydraulic cylinder by adjusting the grinding head speed and reciprocating feed rate. Accurately calculating the processing time of a single piece during mass production can ensure consistency in processing rhythm and accuracy.

4. Honing head and expansion-contraction mechanism

The honing head is connected to the spindle of the machine tool with a connecting rod and is a specialized tool for honing inner holes, made into a cylindrical shape, as shown in Figure 3. The connecting rod is divided into a rigid connecting rod and a floating connecting rod. Due to the deep honing hole, the honing head is connected to the spindle in a floating form. As shown in Figure 3, the floating connecting rod is connected to the spindle floating through the universal cross joint 1. 4 steel balls support the grinding head, and 6 are inserted into the end ball head of the connecting rod 3, which is locked with a nut and hung on the connecting rod and can swing freely at a small angle.

Multiple oil stones are evenly distributed around the circumference of the honing head. During operation, the spindle of the machine tool continuously rotates and reciprocates with it in the hole. Control the expansion and contraction of oil stones through the internal transmission mechanism of the honing head. Adjusting the grinding depth requires radial expansion and contraction of the oil stones on the grinding head, which is one of the difficulties in the design of honing machines. During the machining process, the oil stone of the honing head is fed radially under the action of the expansion and contraction mechanism, gradually grinding the hydraulic cylinder to the required size.

Figure.3 Grinding head and expansion-contraction mechanism

1. Cross joint; 2. Screw; 3. Connecting rod; 4. Push rod; 5. Cone; 6. Steel balls; 7,12. Spring; 8,11. Pins; 9. Oil stone; 10. Oil stone seat

There are two ways for honing oil stones to expand radially: fixed pressure feed and quantitative feed. The vertical honing machine adopts a mechanical transmission precision thread pair oblique wedge mechanism to expand and shrink the oil stone of the honing head. When grinding the surface of the inner hole of the hydraulic cylinder, the oil stone expands quantitatively and applies appropriate pressure to the machining surface.

Expansion and contraction movement is achieved by rotating the screw 2 through the core rod in the hollow spindle, causing the threaded push rod 4 and cone 5 to move, and expanding or contracting the oil stone seat 10 through the inclined surface of the cone. The cone angle of the cone is taken as 45 ° 30 ′, the cone moves 0.5mm, and the oilstone can expand 0.21mm radially. When the push rod and cone move backward, the oilstone 9 and oilstone seat 10 automatically contract radially under the tightening force of springs 7 and 12.

The honing head oil stone is expanded outward and adjusted slightly according to the scale around the handwheel, which can be controlled freely during the honing process of the hydraulic cylinder. The hollow spindle drives the rotational motion of the core rod in the hollow spindle and the additional rotation of the handwheel-driven core rod, which is synthesized by a bevel gear synthesis mechanism and transmitted to the core rod. When the honing head does not require expansion or contraction, the handwheel does not need to drive the core rod for additional rotation. There is no relative movement between the core rod and the spindle, and the direction and speed of the rotation between the core rod and the spindle are the same; When expansion and contraction are required, rotate the handwheel and use a bevel gear synthesis mechanism to generate relative rotational motion between the core rod and the main shaft, thereby causing screw 2 to rotate forward or backward, cone 5 to move forward or backward, and achieving radial expansion and contraction of the grinding head.

5. Honing processing

5.1 Selection of honing parameters

During honing, the contact area between the oilstone and the hole wall is relatively large, and there are many abrasive particles participating in cutting. The main cutting speed V_c of honing is selected to be relatively low, generally below 100m/min, which is only 1/30-1/100 of that of ordinary grinding. The tangential speed of honing is about 15-30m/min for steel processing and can be increased to over 50m/min for cast iron or non-ferrous metal processing.

The average speed V_f of the axial reciprocating feed motion of honing should not exceed 15-20m/min, so the feed rate is about 0.15-0.30mm/r. The main motion and feed motion form a nonrepetitive cross pattern on the hydraulic cylinder, with a cross angle of 30 ° -60 ° being the most suitable.

The width of the precision honing oil stone strip is smaller, while the width of the coarse honing oil stone strip is larger. The honing depth ap generally does not exceed 0.2mm, and each rolling interference is about 0.05-0.12mm.

5.2 Clamping adjustment of hydraulic cylinder and honing head

• 1) Fix the hydraulic cylinder onto the workbench using a specialized fixture, move the workbench to align the honing hole with the spindle, and guide the honing head into the inner hole of the hydraulic cylinder;
• 2) Using a connecting rod, lift the honing head onto a vertical spindle and then introduce the grinding head into the hydraulic cylinder hole. Select different lengths of the spindle connecting rod to determine different introduction strokes;
• 3) Adjust the length of the grinding head stroke and determine the working stroke of the honing head using the stroke stops at both ends;
• 4) Expand the oil stone on the grinding head through the handwheel and press it against the wall of the hydraulic cylinder bore. Slightly adhere to the hole wall, record the handwheel scale value as the reference for adjusting the honing depth, retreat, and feed again, and add a value relative to the reference scale value to obtain the honing allowance.

5.3 Honing operation

• 1) Turn on the coolant to pour into the inner hole and grinding head, start the spindle, and let it move as the main spindle;
• 2) Simultaneously press the axial feed motion button of the grinding head to make the grinding head perform reciprocating feed motion in the inner hole;
• 3) Gradually expand the grinding head through the handwheel for quantitative honing until it reaches the set aperture.

The rotary and reciprocating movements of honing are carried out continuously, and the machining dimensions can be measured online or servo-controlled during the machining process until the honing reaches the set size.

5.4 Hydraulic cylinder cleaning and inspection

• 1) After honing, the grinding head is retracted by the handwheel, and the grinding head and connecting rod are withdrawn from the hydraulic cylinder hole and stopped above the hydraulic cylinder. Press the stop button to stop both the rotation and reciprocating motion of the grinding head, and turn off the coolant switch. Before shrinking the grinding head, remember the hand wheel scale value in case the hydraulic cylinder size is insufficient before honing. It is not necessary to remove the grinding head from the spindle to measure the inner hole of the hydraulic cylinder. If it is necessary to remove the grinding head and then perform honing, it needs to be readjusted.
• 2) Measure the hydraulic cylinder barrel. Move the workbench, clean the hydraulic cylinder, and inspect the hydraulic cylinder. If the size and shape accuracy of the hydraulic cylinder are qualified, the entire honing process is completed.

The surface after grinding is not allowed to have abrasive knife marks, and the grinding head oil stone should be corrected every time it is re-grounded. It is best to grind the oil stone surface into an arc close to the aperture. If the honing accuracy and roughness cannot meet the requirements, it is necessary to adjust the honing cutting amount and stroke to improve the grinding effect. PLC and touch screen control the electrical system of the machine tool, and the entire machining process of the hydraulic cylinder is manually loaded and unloaded to achieve semi-automatic circulation.

6. Conclusion

Honing has the advantages of precision grinding, high processing efficiency, controllable operation, and convenient adjustment. With the rapid development of mechanical manufacturing in our country, the requirements for the machining quality of parts are becoming increasingly high. Not only does it require high precision, but it also requires low surface roughness. Due to the unique processing advantages of honing inner deep holes, through holes, or holes, it can be widely used in precision parts processing fields such as precision machinery, engineering machinery, aerospace, automotive, and motorcycle industries. It can meet the needs of rough honing and precision honing of inner holes in precision parts: the pressure adjustment range can be from less than 0.1MPa to 2-3MPa or above, and it can achieve precision machining from low-pressure precision honing with small feed to high-pressure rough honing with large feed. The combination of rough honing and fine honing can not only increase machining allowance, improve machining efficiency, and shorten the machining cycle but also greatly improve machining accuracy.

Author: Wang Jianzhong