Hydraulic Fitting Assembly Guide

Using 'angle of rotation' to predict bolt and fitting torque tightness

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Learn what affects the torque during assembly of hydraulic fittings

Warning Notice

Recommended tightening procedures can vary between manufacturers and following the appropriate procedure is vital to ensure safe, leak-free operation and long service life.

This fitting assembly guide has been designed to provide published and calculated data to help identify the correct assembly procedure for specific fittings and check they fall within an appropriate stress range for your application. We also review a number of tightening issues that can arise due to variations in lubrication, temperature, surface finishes.

The calculator suggests which angle of rotation can be used when there is insufficient space for a torque wrench. Users must always check the latest supplier recommendations for their particular application and as best practice, we recommend testing ‘Torque VS Angle' values for each batch and size of fitting.

Rotating a fitting through a specific angle can very quickly overstress the component if the connection is not able to bed in further or properly flex. Never exceed the material yield force torque limit for each particular size of fitting.

How hydraulic fitting torque and angle of rotation works

The torque vs angle guide provides assembly details for standard bolts and modern hydraulic fittings.

It calculates the maximum permissible torque e.g. the torque predicted to reach material yield point and then provides the angle of head rotation required to achieve this. On bolts this assumes that the free length of the shaft stretches as the bolt is tightened.

For fittings the calculator estimates the force that will be induced due to the fluid pressure and its equivalent angle of rotation or torque (depending on the relevant friction factor). The fitting tightness should usually lie above the highest force experienced in service but at or below the yield point of the material.

Users must select the correct material that their bolts or fittings are made of.

Many older style fittings are not listed as manufacturers recommend special pre-assembly machines, techniques and training.

Factors affecting correct thread tightness

Screw threads should be tightened such that the internal stress is greater than the maximum stress experienced during service. The will ensure that the internal stresses does not keep oscillating and therefore induce fatigue failures. It also ensures that the bolt remains tight so will not work loose.

The internal stress is directly related to the elongation of the free length of a bolt or fitting shaft. By rotating the shaft a set number of degrees against the thread pitch, we can accurately determine the internal bolt stress. Friction may still affect this as it can cause a different angle of rotation at the bolt head to the thread. Therefore stopping the shaft elongation.

When torque tightening a bolt or fitting, friction can also have a significant effect. A certain percentage of the torque is always absorbed by friction and with unlubricated stainless steel, for example, this can be very high, but a completely unknown value.

Stainless steel threads should always be lubricated but we do not advise the lubrication of non S/S threads on adaptors threads, on the fluid side of the seal. This is because experience has shown that airborne contamination is like to adhere to the lubricated thread and result in more serious reliability issues than from a poorly torqued fitting.

Temperature will also have an effect on thread tightness because material properties change with temperature. The thread tightness guide is based on 20C ambient but you can easily change the material properties inside the program, if your application is subject to significant temperature extremes, of different materials.

Commonly quoted reliability figures for the different tightening methods are:

Operator "Feel" +/- 35%

Torque Wrench +/- 25%

Angle of rotation of nut +/- 15%

Three Stages of Thread Tightening

There are three potential stages for assembling a hydraulic fitting. Pre-assembly is required for some fittings before they are fitted to the equipment. All fittings require a particular fitting procedure the first time they are used but some fittings will also require a different approach when they are broken and re-made.

Incorrect assembly is known as a significant cause of leakage and occasionally it does cause fitting failures and potential injuries. Modern fittings have generally been designed to reduce the need for complicated assembly procedures and therefore have significantly reduced the risk of operator error during assembly and remake.

Pre-Assembly Requirements

The first time a thread or fitting is put under stress there will be some distortion of the faces or form. There will always be some small manufacturing or surface finish issues between the two mating thread surfaces.

Many fittings, particularly the older designs, require a pre-assembly procedure which may include crimping or bedding parts of the fitting. This is often achieved via a specified angle rather than a given torque, however, there are differences between the approach from different manufacturers and different fitting types.

Assembly Requirements

First off assembly is either after the pre-forming operation or when the fittings are assembled for the first time such that the fitting threads and mating faces have not bedded in.

Assembly instructions vary between different manufacturer and products so users must pay attention to the instructions specific to their component and suppliers. When comparing the manufacturer instructions it's clear that some fittings are expected to distort significantly during assembly although it's not clear if this is the first or every time the fitting is tightened. We recommend marking the final assembly tight point so that the fittings are not overtightened on remake. Users must never exceeding the fitting's yield torque.

Re-make Requirements

The procedure required for re-making a connection a second, or greater number of times, may be different for each fitting or manufacturer. Users must be aware that applying the same angle of rotation as specified for the assembly procedure could result in the connection being over-stressed to a point that could risk failure.

Tips for selecting and maintaining hydraulic fittings

Always use the latest designs of modern fitting wherever possible. Older designs risk more errors, extra leakage and potential injury.

Never go above the maximum torque for yield. Often bolts are only recommended to be tightened to 75% yield but this depends on the application. The tightening guide tool provides you with both the maximum yield and a minimum based on potential system pressure peaks. The safe region lies somewhere between these two.

Assemble to first off assembly marks, where available, or add them if they are not there. The angles of rotation specified by the fitting manufactures allows for some distortion of the fitting, which may not be as high during later reassembly.

Before assembling any fitting the user should check that the fitting is clean and free from burrs, damage or surface manufacturing errors.