Hydraulic cylinder tubes are available in three bore-finish grades: NBK (as-drawn), SRB (skived and roller-burnished), and honed. Each finish serves a different performance tier, and specifying the wrong one means either paying for precision you do not need or installing a tube that will not seal properly against your piston and rod assemblies.
NBK: As-Drawn Finish
NBK (Normalisiert, Blank, Kaltgezogen — normalized, bright, cold-drawn) is the base-level hydraulic tube finish. The tube is cold-drawn through a die and over a mandrel, producing a smooth internal bore surface with typical roughness values of Ra 1.6–3.2 μm. Bore tolerance is generally within ±0.15–0.25 mm on tubes up to 80 mm ID, which is adequate for applications where the bore will be further machined, plated, or lined before cylinder assembly.
NBK tubes cost 25–40% less than SRB or honed equivalents, making them the economical choice for prototype cylinders, low-pressure pneumatic applications, non-critical actuators where leakage past the piston is acceptable, and any cylinder where the bore will receive secondary processing. Major mobile equipment OEMs typically purchase NBK tubes when they have in-house honing capability.
SRB: Skived and Roller-Burnished
SRB processing removes a thin layer of the bore surface by skiving (a precision boring tool) and then roller-burnishes the exposed surface with hardened rollers under pressure. The burnishing action plastically deforms surface peaks into valleys, producing a bore finish of Ra 0.4–0.8 μm with a work-hardened surface layer approximately 10–20 μm deep. Bore tolerance tightens to H8 (±0.025–0.050 mm for typical cylinder sizes).
SRB is the volume production standard for hydraulic cylinders in mobile equipment, industrial presses, and machine tools. The combination of good surface finish, tight bore tolerance, and work-hardened surface provides reliable piston and rod seal performance without the cost premium of honing. Approximately 60–70% of production hydraulic cylinders worldwide use SRB tubes.
Honed Bore: Premium Finish
Honing uses abrasive stones rotating and reciprocating inside the bore to produce a crosshatch surface pattern with Ra values of 0.2–0.4 μm and bore tolerances to H8 or tighter. The crosshatch pattern — characterised by intersecting grooves at 30–60° — serves a specific tribological function: the grooves retain a thin oil film on the bore surface, providing hydrodynamic lubrication between the bore and the piston seal. This reduces friction, extends seal life, and minimises stick-slip at low speeds.
Honed bore tubes are specified for servo and proportional hydraulic cylinders where position accuracy and speed control demand minimum friction and zero stick-slip, high-cycle applications where seal life is critical to maintenance intervals, large-bore cylinders (above 150 mm) where the consequences of premature seal failure are costly, and any application where the cylinder specification explicitly requires a honed bore.
Surface Roughness vs Seal Performance
The relationship between bore roughness and seal performance is not simply “smoother is better.” An excessively smooth bore (< Ra 0.1 μm) does not retain enough lubricant film to support hydrodynamic lubrication, leading to boundary contact, increased friction, and accelerated seal wear. The optimal roughness range for most elastomeric piston seals is Ra 0.2–0.6 μm with a plateau-honed profile. SRB falls within this range for most applications; honing provides the additional control of surface lay pattern that servo-grade cylinders require.
Making the Right Choice
The decision framework is driven by three factors: the cylinder’s pressure and speed regime, the required seal life between maintenance intervals, and the cost sensitivity of the application. For prototype and low-volume production, NBK with in-house machining offers flexibility. For volume production of mobile and industrial cylinders, SRB delivers the best cost-performance balance. For high-precision servo applications and large-bore cylinders, honed bore is the professional specification.
Need hydraulic cylinder tubes? Specify your bore size, finish, and tolerance requirements — we quote NBK, SRB, and honed bore from stock and production schedules.