What a hydraulic cylinder assembly contains

A hydraulic cylinder converts fluid pressure into linear force. The complete assembly is more than a tube. It normally includes a barrel, a piston rod, a piston, end caps such as a gland and a base, seals, ports and a mounting interface. Each part has its own tolerance chain, and the assembly only works when those chains line up.

A barrel-only enquiry is a different scope from a finished assembly. Some buyers ask a supplier to hone a tube, while others want a supplier to deliver a tested, leak-free cylinder ready to bolt into a machine. The contract manufacturing service covers welded and machined fabrications built to customer drawings, but the assembly boundary, testing and certification must be defined before a quotation is useful.

Drawings and bill of materials the supplier needs

Send the controlled drawing and BOM for every component, not only the barrel. Useful inputs include:

  • barrel inside diameter, outside diameter and length, with the bore tolerance stated as a fit class such as H8 or H9;
  • barrel material and grade, for example ST52, E355 or CK45, supplied against a material certificate such as EN 10204 type 3.1;
  • piston rod diameter, material, surface hardness and hard-chrome thickness;
  • piston dimensions and the seal groove geometry;
  • end-cap and gland geometry, thread forms and port size;
  • mounting style, pin centres and any sensor or cushioning provisions.

Precision cylinder tubing is usually specified to standards such as EN 10305 for cold-drawn or cold-finished seamless steel tubes, with bore geometry referenced to ISO 286 limits and fits. Naming the standard in the enquiry prevents the supplier from quoting a commercial tube that cannot hold the finished bore.

Welding scope and welding code

End caps, glands and ports are often welded to the barrel. The weld quality should be tied to a code rather than left to the supplier. AWS D1.1 is a common structural welding reference, while ISO 3834 defines quality requirements for fusion welding of metallic materials. Ask the bidder to state the welding procedure specification and procedure qualification record basis, the inspection level, and which welds receive non-destructive testing.

Define who owns the welding procedure. If the buyer's design controls the joint, the supplier executes it. If the supplier proposes the joint, the buyer should approve it before fabrication. This boundary matters because a change in joint preparation can move the responsibility for fit and strength.

Machining and surface finishing

The barrel bore is normally finished by honing or skiving and roller burnishing to reach the stated surface roughness and diameter tolerance. The piston rod is machined, ground and hard-chrome plated, then ground again to size. Seal grooves must be cut to the seal manufacturer's dimensions, because a groove that is a few tenths of a millimetre off will leak or wear early.

State the finished bore tolerance, the rod diameter tolerance and the surface roughness explicitly. Do not ask the supplier to "make it smooth." A measurable value lets two bidders quote the same part and lets inspection confirm it.

Assembly, sealing and pressure testing

Seals should be selected against the fluid, temperature and pressure in the buyer's application. The buyer normally specifies the seal compound and profile, while the supplier assembles in a clean condition and installs the parts without nicking the rod or scoring the bore.

Pressure testing is where scopes diverge most. A hydrostatic or leak test can be done at the supplier's works, but the test pressure, hold time, acceptance basis and certification must be written into the RFQ. ISO 10100 describes acceptance tests for hydraulic cylinders and is a useful reference for the test method, not a statement of any supplier's standard. If the buyer needs a recorded test with traceable results, say so in the enquiry.

Inspection evidence and material certificates

Ask for a document package that matches the risk of the application. At minimum this usually includes material certificates for the barrel and rod, a dimensional inspection report against the drawing, a weld map where relevant, and the test record if a pressure test was required. EN 10204 type 3.1 certificates are common for the main pressure-containing parts.

The supplier should not invent design values. If the buyer's drawing sets the bore tolerance, the supplier reports measured values against it. If the buyer leaves a value open, the supplier should ask rather than assume a convenient number.

What to send with the RFQ

A useful hydraulic cylinder assembly enquiry contains the controlled drawings and BOM, the material and certificate requirements, the welding code and inspection level, the bore and rod tolerances, the seal specification, the test requirement, the target quantity and the delivery destination. Ask each bidder to return an assumptions and deviations list so the comparison stays on the same scope.

Realjet produces welded and machined machinery components to customer drawings and specifications. Send the barrel and rod drawings, the welding and finishing requirements, and the test scope through the contract manufacturing enquiry route so the response addresses the correct fabrication and assembly boundary for the project.