Hydraulic valves, piston pumps and cylinders require increasingly close fits between the piston or spool and bore to reduce internal leakage. The components also need precision surface finishes for high performance and long seal life.
That’s according to Phil Hanna, product manager for machines/gages at Sunnen Products, St. Louis. Hanna offered these observations in a presentation, “Bore Finishing for Hydraulic Components,” part of the virtual June NFPA/FPIC Regional Conference, which focused on improving fluid power systems with precision manufacturing.
Engineers can achieve design goals using several different bore finishing techniques, and all are secondary machining operations, said Hanna. These include honing; and skiving and roller burnishing.
Honing is a secondary abrasive machining process which follows hole creation to improve bore geometry and surface, he explained. Honing is an effective alternative to internal grinding, precision and hard turning, and reaming. It can correct numerous unwanted conditions, such as bores that are out-of-round, wavy, tapered or undersized, or have residual reamer marks. But honing cannot change the centerline of the part.
Typical honing capabilities include diameters from 1.52 to 1,524 mm (0.060 to 60 in.) in bore lengths to 14.5 m (47 ft) or more. And the process is suited to a wide range of materials, from steels, aluminum and titanium to carbide, glass, ceramics and plastics.
Honing can create a precise surface finish. It can control size down to 1.27 µm (0.000050 in.), geometric tolerances for roundness, straightness and cylindricity ≤ 2.03 µm (0.000080 in.), and surface characteristics such has Ra down to 0.05 µm (2 µ-in.) and Rz down to 0.4 µm (16 µ-in.). And the process can produce crosshatch angles from 20° to 120° inclusive, ±1°. Usually, required surface characteristics follow recommendations by seal manufacturers in hydraulics applications.
The process itself can involve single-pass or multi-stroke operations. In the multi-stroke honing process, either the tool or part must float. The tool rotates, and it or the part moves back and forth. In the single-stroke process, the tool rotates and pushes through the bore. It might require a series of tools, depending on stock removal, geometry and surface finish requirements.
Multi-stroke honing produces a crosshatch surface texture with good bearing surface and oil retention. Single-pass generates a tight, helical surface texture pattern. Successful implementation also requires proper fixturing, tooling, abrasives and cutting fluid, in addition to the base machine. The tooling works together with abrasives and cutting fluids, of which there are many. Sunnen, for example, offers around 7,250 different tooling parts, 21,000 abrasives and 19 different coolants/lubricants.
Skiving and roller burnishing processes were developed in the 1970s as an alternative to honing tubing for hydraulic cylinders. It is an extremely fast process when compared to honing, in the neighborhood of 70% to 80% faster, Hanna explained.
It is capable of holding bore diameter tolerances to 0.025 mm (0.001 in.) The necessary equipment requires a large up-front investment, which makes it suitable for high-volume tubing production.
The skiving and roller-burnishing operation, as the name implies, actually involves two processes in one tool. The tube is end clamped in the machine and the tool advances inside. Coolant is pumped through the ID of the tube, flows around the tool and pushes chips out of the front of the tube during operation.
The first part of the tool contains carbide cutters that rotate and advance axially to remove material. This section performs the skiving. The second part of the tool contains hardened rollers that deform the peaks creating by skiving and produce an extremely smooth surface finish, thus roller burnishing.
The process offers very fine surface finish, such as Ra = 0.05 to 0.20 µm. While roller burnishing generates a mirror-like result, however, there is not precise control of the surface finish.
In addition, burnishing creates a residual stress layer in the cylinder wall that improves fatigue life. Surface hardness increases approximately 50% in steel tubing.
For high-volume hydraulic cylinder production skiving and roller burnishing is the preferred process today, said Hanna. Honing is certainly still a viable alternative for low-volume work. It’s not unusual for cylinder manufacturers to use multiple skives and one or two hones, depending on the output demands. Honing is also well-suited for low volume off-size work, such as repair or refurbishing.
Filed Under: Cylinders, News