The problem occurs, when the rod end port or line of a single rod double acting cylinder is restricted and system pressure is applied to the closed end.
A Few Fundamentals: Force Area Pressure Force is expressed in lbs. Area is expressed in in² Pressure is expressed in PSI
The FORCE of the cylinder in lbs. Is the system pressure multiplied by piston area…remember this area is round The area of a circle is: A = π r² Lets assume this cylinder is 4” in diameter The area of this PISTON is 3.14 x 4 or 12.5 sq. in Lets assume the system pressure is 1000 psi This cylinder WILL produce 1000 x 12.5 or 12500 lbs of FORCE! This area is known as the major area
If Force = Pressure x Area then Pressure = Force ÷ Area With this 4” diameter cylinder, the rod diameter is 2” We must subtract the rod Area from the Piston Area Rod Area is 3 in² To obtain the MINOR Area! 12.5 - 3 = 9.5 in² Major Area is 12.5 in² Minor Area
X When system pressure is applied to the closed end and the rod end is restricted, pressure intensification occurs Pressure = Force ÷ Area Area = 9.5 in²Pressure = 1000 PSI 12500 ÷ 9.5 = ?? 12500 lbs of Force
1000 PSI 1325 PSI 1325 PSI is within the rod end of this cylinder when the rod end port is restricted and system pressure is applied to the closed end of the cylinder!
5 “ cylinder 4 “ rod working under 2200 PSI Major Area = 19.6 Rod Area = 12.5 Minor Area = 7.1 Force = 19.6 x 2200 or 43120 lbs Intensified Pressure = 43120 ÷ 7.1 or ?? X
2200 PSI 6073 PSI What could cause a rod end port to be restricted? Would you like to be working near this cylinder if this condition existed? Where might you find this type of cylinder?
Types of restrictions include: Hose restriction\complete blockage Hard pipe damage Ball valve or gate valve INSTALLATION Foreign objects/contamination What can WE do to minimize exposure to this HAZARD?
Be thorough when making Equipment Examinations NEVER use a ball valve for controlling a hydraulic cylinder circuit Don’t adjust any circuits’ pressure relief valve you may be setting yourself up for disaster Repair or replace damaged components immediately Don’t wait until the maintenance shift
A mechanic with no formal training in hydraulics was attempting to test a hydraulic boom lifting cylinder for leaks while attached to a large front end loader. He used a uni-directional (one way) inline flow meter. He installed the flow meter in series with the cylinder between the cylinder and the directional control valve in the rod end hose. Asking the operator for assistance, the mechanic instructed the operator to retract the cylinder until it stalled against the base of the cylinder and to hold full pressure. The mechanic observed the flow meter and saw no indication of leakage. He signaled to the operator to extend the cylinder a short distance to repeat the test.
Uni-directional Flow meter To directional control valve (valve bank)
As the cylinder began to extend there was a loud explosion. At the same instance the flow meter assembly, still attached to one end of the hose, came flying across the machine and skimmed across the top of the mechanics head. The hose end had ruptured at the crimp connection near the flow meter. Hot oil escaping from the severed hose sprayed his face and neck. He was treated for minor burns. He narrowly escaped Disaster He did not understand that a uni-directional flow meter contains an internal check valve
Hydraulic accidents frequently occur and for the most part are not understood. Treat hydraulic systems with the same respect you would electrical systems. They ARE as potentially hazardous.
Colorado Division of Minerals And Geology Mine Safety Training Program