All hydraulic pumps and motors have threaded fasteners.
This can only mean one thing - when you
open a pump or a motor you unscrew stuff!
Although it seems like an easy task - I mean, come on, how difficult
can it get? Just turn them screws counterclockwise hard enough, and that's it!
(and, in fact, it IS in most cases) - there are still those
"stubborn" screws which, if not unscrewed properly, can become a
pain in the ass! Since I get to see many shops, every now and then I get to see a
mechanic unscrewing something hydraulic depressingly wrong - a sight I try to avoid
at all cost, and the main reason this article saw the daylight...
So, why do some screws offer so much resistance? In the case of our
hydraulic pumps and motors there are two probable explanations
- either a screw has been there for a long long time, or some kind of
thread glue (loctite) had been used to secure it.
That is when the Double H technique kicks in. Double H standing for Hit and Heat. Whenever you come across a tight screw, before going
nuts with the torque, you must Hit it or Heat it, or both, depending on
the severity of the unscrewableness. Vibrations and thread material
deformation, caused by hitting, and thermal shock and expansion, caused
by heating, destroy microscopic bonds inside the thread, making the
unscrewing a much easier job.
The rule of thumb is - if some bolt
doesn't come out, most probably you are not Hitting or Heating it hard
enough! Provided, of course, that you use enough torque and grip. I know that
for many people this is not news, but from what I have seen, there are
still plenty of those, who prefer wrench bending to smart unscrewing.
A separate word must be said about glued threads, the
use of which is common in hydraulic pumps and motors. Your best
friend here is, yet again, heating, because high temperature softens the
glue. Depending on the type of the glue used to lock the thread, it can
be next to impossible to unscrew a bolt without heating the joint
Take, for example, the piston retainer from a Rexroth
A6VM motor, which is held in place by 14 small glue-locked screws.
Original screws are class 12.9, alloy steel quench and tempered. These
are very strong, but when over-torqued they are very easy to fracture.
I have seen many of those break when the heating is omitted, and
must tell you that taking out the screw left-over from the female
thread is all but easy.
Another very important thing many (even
experienced) mechanics do not understand is HOW the glued thread should
be heated. The most important thing here is to make sure the whole
thread is heated (normally to around 100-120 C), which with massive
parts, can only be achieved by heating THE WHOLE part. For the above
mentioned A6VM motor it would mean heating the whole shaft.
The most common mistake is applying local heat (with
a torch) to screw heads. I even heard once an experienced
technician teaching a greenhorn to PUT THE SCREW HEADS RED (!!!) with a
torch to disassemble the poor retainer. It might work, but applying
small concentrated amounts of heat doesn't guarantee heating of the
whole joint, and, as the massive steel shaft is a good heat conductor, there's a risk that
the glue inside the thread won't be affected. The correct
way is to heat the whole upper (piston holding) part of the
shaft equally and evenly (although the original
rexroth repair manual recommends the use of a hot oil bath, the torch works just as good)
until it reaches the 100-120 C softening point. Then the screws will
come out nice and easy.
The pictures to the left are just an example
of such local heat madness. The bluish color of the retainer shows that
the mechanic did apply the heat, but only to the top of the screw heads,
which wasn't nearly enough to undo the glue inside the threads. The fact that
he broke so many of those screws states that he is, most probably,
stubborn as a donkey.
So, whenever you find yourself fighting with a stubborn threaded joint - remember - Double H will come to the rescue!
Something isn't coming out? You are not Hitting or Heating it hard enough!
Need to take out a loctite locked screw? Heat the whole part, not just the bolt head!
Following these simple rules will make unscrewing of
"stubborn" threads a piece of cake, althought (unfortunately) there will
always be a small percent of them bolts which you will HAVE to drill
out or cut out...