1997-09-08

Wire Braid Angle Response Characteristics in Hydraulic Hose 972706

This report is concerned with the effects of braid angle on the behavior of hydraulic hose. For equilibrium conditions to exist, and if the braid layers are assumed to bear tension forces only, the angle of the reinforcement layers must be along that of the total force exerted by the internal pressure. This is the neutral angle θN, which has a theoretical value of 54.74° (54°44′).
It is possible to hypothesize a fretting wear model in which wires move on top of one another inside a braid layer if the braid angle is different from this theoretical neutral angle. Even though theoretical claims are made by some technical professionals, the hydraulic hose industry has been successfully making hoses with non-neutral braid angles for years. Testing and application have shown that fretting wear is not a principal cause of hose failure and fatigue. In fact, experimental case studies demonstrate that hoses made with non-neutral braid angles perform as well as hoses made with the neutral braid angle.
It is common industrial experience for hose to fail at peak pressures that are less than the expected value. One important cause of this premature failure is lack of uniform load distribution between the inner and outer braid layers. In this case, the inner braid carries more than its equal share of the load and it reaches the fatigue limit while the outer braid is only slightly stressed. There is, however, a method for accommodating the unequal loads between two layers of a braided hose. This is achieved by laying the inner and outer braids of a two wire braid hose at angles that lie symmetrically on either side of the equilibrium helix angle of 54.74°.
Other considerations to make with respect to early hose failure are fitting retention, securement and minimum bend radius of the hose, among others.