The shape of this impulse curve is strictly controlled to ensure standardised, consistent test results.
The rapid rise and fall of the pressure increase and decrease cycles must fall within the shaded limits.
This happens at between 30 and 70 cycles per minute, with 60 cycles per minute being a typical frequency. This means that it only takes only one second for one impulse cycle.
Example impulse test configurations
For example, RYCO T3K4D: SAEIOOR17, 1/4″ one wire braid hose with a maximum working pressure of 210 Bar (3,050 PSI) and maximum working temperature of +1000C would be tested as follows:
Maximum Impulse Pressure: 210 Bar x 133% = 280 Bar (4,060 psi)
Temperature: + 1000C
Minimum Bend Radius: 50mm in 1800 arc
Number of Impulse Cycles: 200,000
Four ‘unaged’, or new, hose assemblies must be tested, and all four must pass the impulse test. If any one assembly fails, the test is failed. It is a tough test.
The rapid pressure increase is like hitting the hose with a sledgehammer, each second for more than 55 hours! It is even more difficult for spiral reinforced hoses.
Spiral wire reinforced hose testing
For SAE IOOR12, R13 and R15 the number of impulse cycles increases to a minimum of 500,000.
That is almost 140 hours to complete the test at one cycle per second!
For example, 1″ SAE 100R12 four spiral wire reinforced hose with an SAE maximum working pressure of 276 Bar (4,000 PSI) and maximum working temperature of + 121 c would be tested as follows:
Maximum Impulse Pressure: 276 Bar x 133% = 367 Bar (5,320 PSI)
Temperature: + 1210C
Minimum Bend Radius: 305 mm in 900 arc
Number of Impulse Cycles: 500,000
Additionally, if the hose manufacturer claims higher temperature or higher working pressure capabilities, the hose couplings must be tested according to the higher levels.
Impulse test curves
This graph shows impulse test curves for hose types that meet the SAE pressure on the left, and for RYCO products on the right.
Note that the test pressure for R12 hose is higher for the RYCO product.
This is because the Maximum Working Pressure is higher than standard, and consequently the impulse test pressure is higher.
Four hoses on impulse test
When impulse testing, the hoses appear to become tighter, and then release back to their previous position.
The point where they are tight is the high pressure phase of the impulse curve.
It should not be assumed that spiral reinforced hose assemblies will last for only 500,000 impulses in use, or that one or two wire braid hose assemblies will last for only 200,000 impulses.
During the impulse test, all the working conditions of the hose are at the highest and most demanding levels.
In typical applications, working conditions are nowhere near as severe as an impulse test, so in use, the hoses may last longer than the number of impulse cycles that the SAE and / or DIN specifications require.
Using Pressure-Impulse Curves
(P = Pressure, N = Number of impulses )
This logarithmic graph can be used to estimate the number of impulses a given hose assembly will withstand at given pressures.
This point is where a hose assembly is expected to fail at its Minimum Burst Pressure (MBP), which is 400% of its Maximum Working Pressure (MWP).
As the assembly is expected to fail at this pressure, it can be considered to fail after one impulse.
This point is where the hose assembly is expected to achieve the specification requirements for that hose type. In this case, 200,000 cycles at 133% of Maximum Working Pressure (MWP).
The PN curve can be used to estimate an accelerated impulse test.
For example, if impulse tested to 200% of Maximum Working Pressure instead of 133%, the hose assembly can be expected to reach only 2,000 impulses instead of 200,000.
The PN curve can be extended to estimate the expected impulses at or below Maximum Working Pressure.
However it can not be expected to be any more than a ‘rule of thumb’ as the number of expected impulses becomes so high it becomes less likely to be to achievable or even possible. At this point, the hose will be affected by ageing before mechanical fatigue.
Factor of Safety (FOS)
The 4: 1 factor of safety applies to the hydraulic hose assembly and not to the connector terminations.
These terminations have the same factor of safety that is shared by all other components of the hydraulic system i.e. 2.5:1
Hose assemblies require a higher factor of safety because being flexible, they are subjected to more dynamic stresses and influences than other system components such as valves, actuators and tubework.