August 1977
DUMP HOIST CYLINDER AIR BLEEDER VALVE
1. A user from the field has submitted an EIR on the dump hoist cylinder air bleeder valve of the CCE-IHC Paystar
F5070, 71,000 GVWR Dump Truck. The user complained that it was much too easy to strip the threads on the air bleeder
valve cap at the top of the dump body telescopic hoist cylinder, enclosed within the dog house. Pliers had to be used to
get the cap tight enough so the valve would not leak. The user also recommended substituting a 90 degree twist handle,
radiator drain type, brass pet cock in the 1/8 inch NPT, cylinder boss, in lieu of the existing bleeder valve. The reasoning
was that the pet cock would make it so much easier for personnel to bleed the air trapped in the hoist cylinder with only a
quarter turn of the handle.
2. Air gets trapped in the top of the hydraulic hoist cylinder and causes spongy, jerky, uncontrolled dump body hoist
operation. This air compresses like a spring. When the friction of the packing of the three stage telescopic hoist cylinder
is overcome, the body pops up and then binds again, until pressure build-up repeats the "up and hold" cycling. Dump
body decent to the truck frame occurs in a series of starts and stops, until it slams against the truck frame. Since air is
compressible and hydraulic oil is not, dump truck users must get all of the entrapped air out of the hydraulic components
for smooth dumping operations. To get the air out a bleeder valve is installed at the hydraulic system's highest point, the
top of the body hoist cylinder.
3. Naturally, bleeding this entrapped air out of the hydraulic system is a must, but even more important is keeping the air
out of the hydraulic system in the first place. This entrapped air gets into the hydraulic system from the suction side of the
pump and from the hydraulic reservoir oil tank. Limit the source of air, first.
4. In covering the suction side of the hydraulic pump, inspect the reservoir tank-to-pump lines and components to make
sure that they are air tight. Because of normal outside air pressure (14 psi) and the vacuum of the hoist pump, this
suction line will allow air from outside the system to slip past the tiniest opening, one that would not leak oil out of the
opening. To check this source of hydraulic system air, you should do the following:
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