How is a hydraulic hose assembly made? Skiving, crimping and checking the crimp

A hydraulic hose assembly is made in a fixed order: the hose is cut to length, skived at the end if the fitting calls for it, fitted, crimped and measured. Then it is cleaned, capped and, where required, pressure tested. Skive length, die set and crimp diameter are not the fitter’s choice; they come from the hose and fitting maker’s instructions.

What are the steps in making a hydraulic hose assembly?

Making a hydraulic hose assembly starts with checking the parts and ends with a test; in between come cutting, skiving, fitting and crimping. MDG 41, the fluid power guideline written for mines by the NSW Resources Regulator in Australia, lists what a hose shop’s written procedures should cover:

1. Material verification.
2. Cutting. Cut length, cutting, cleanliness.
3. Skiving or buffing. Then cleaning.
4. Coupling assembly. Depth, lubrication, verification.
5. Crimping.
6. Final inspection.
7. Pressure testing, where required.
8. Quality plan.

One condition comes first: only “matched” hose and fittings are used. An assembly with non-matched fittings, the guideline says, may reach burst pressure and still most likely fail impulse testing. The assembly is rated by its lowest-pressure component; for the hose, see our article on pressure ratings.

How is the hose cut, and how is the length worked out?

The hose is cut square to its axis, and the cut length has to allow for what the fittings add. A 1980 US Army technical manual for a hydraulic repair workshop (TM 9-4940-468-14) notes: “keep in mind that the couplings will add length to the completed hose assembly”. MDG 41 gives ±1% as the general tolerance on overall length.

The manual has the hose held firmly, square to the saw blade and slightly curved, so that it pulls away from the blade without binding.

Which hose ends need skiving?

Skiving means removing the rubber cover at the end of the hose down to the wire reinforcement. By the US Army manual’s rule it is skipped only when both hose and fitting are marked no-skive; every other application requires it. The reason:

“Hose end preparation (skiving) involves taking the rubber cover from the hose, near the end. This is done so that the hose coupling can grip the wire reinforcement.”

US Army, TM 9-4940-468-14, 29 August 1980

The manual describes screw-together couplings, but MDG 41 lists skiving among the steps for crimped assemblies too.

Some hoses are also skived on the inside. NACG, a Canadian heavy construction and mining contractor, sets the order in its workshop procedure: outside first, because the mandrel that supports the hose during external skiving cannot do so once the inner rubber has gone. Its aim: “The object is to remove as much rubber as possible without damaging the wire reinforcement.”

How do you crimp hydraulic hose fittings?

To crimp a hydraulic hose fitting, the ferrule is squeezed onto the hose between the dies of the crimper until the machine reaches the diameter it was set to. The fitting must be fully home first.

Any lubricant used, MDG 41 adds, should be compatible with the system or inert. NACG’s crimping procedure tracks the insertion depth with a paint-marker line on the hose: after crimping, the line must still touch the end of the coupling.

Die set and setting come from the crimper’s manual. In the Finn-Power operating instructions each die set is named by the smallest diameter it closes to, and a dial adds the rest: for a 20.6 mm crimp, fit the 19 mm die set and turn the dial to 1.6.

DGUV, the German statutory accident insurance body, recommends crimp fittings in its 2017 rule on hydraulic hose lines (Regel 113-020) and wants screw-type ones avoided as less safe to assemble. A company making its own assemblies may use only devices approved by the fitting manufacturer, and no used hose or fittings.

How is the crimp diameter measured?

The crimp diameter is the outside diameter of the ferrule after crimping, and it is measured on every end. NACG’s procedure, once the dies have opened: “Measure fitting A with digital calipers or use a no-go gauge.” The reading must be within the tolerance of the finished crimp diameter; the ferrule is then checked by eye for uniformity and crimp length.

The crimper’s dial does not replace the measurement. The Finn-Power manual gives the scale an accuracy of +/- 0.1 mm under the factory calibration condition, not counting the elastic recovery of the fitting. Where a fitting needs more pressure, the machine deflects and the diameter can come out larger than the scale value. The manual’s answer: a test crimp, a measurement and a corrected setting.

DGUV wants the assembler to be competent to check the crimp because, in our translation, “the safety of people depends on the quality of the crimp and on the check that follows”.

MDG 41 names a wrong machine setting “resulting in over- or under-crimping” and a worn die among the causes of a fitting that blew off. For how they show in service, see our article on hose failure signs.

How is a hose assembly cleaned and tested?

Cutting and skiving leave rubber and metal debris in the hose, which has to come out before the fittings go on. MDG 41: “A cleaning projectile should be shot through the hose in both directions before assembly of the hose ends.” The finished assembly is capped.

MDG 41 does not demand a pressure test for every assembly. For those rated high risk it asks for a proof test at two times the maximum working pressure, held for 30 to 60 seconds, with a compatible fluid, not compressed gas. DGUV recommends agreeing an individual test where the application’s risk calls for it.

What should you check when a finished hose assembly arrives?

On a finished hose assembly, check five things: marking, fittings, hose, cleanliness and the order. DGUV recommends buying hose lines ready-made from a specialist shop and checking them on receipt; the marking and order points come from its rule.

The other three follow MDG 41’s visual inspection before dispatch. The guideline writes it for the shop that made the assembly; using it at goods-in is our inference.

1. Marking. Under the DGUV rule every hose line carries the maker’s name or mark, the maximum working pressure and the year and month of manufacture; they can be stamped into the ferrule during crimping.
2. Fittings. Securely crimped and right for the hose size and type; no cracks, distortion, rust or bulges. Do not judge the type by the thread: see fitting and thread types.
3. Hose. No kinks, bulges, soft spots, cuts or protruding wires.
4. Cleanliness. Clean inside, capped at both ends; MDG 41 wants non-metallic caps.
5. Order. Order specification met, recommended storage time not exceeded; the certificate for any agreed individual test is there.