dfkrug
Super Member
- Joined
- Feb 3, 2004
- Messages
- 7,200
- Location
- Santa Cruz Mtns, CA
- Tractor
- 05 Kioti CK30HST w/ Prairie Dog backhoe, XN08 mini-X
Well, some would say, never. And terminology does matter.
I am prompted to write this after getting one of those emails from our hydraulics guru from
Down Under, Bendan Casey. In it, he equates a "ram" to a "displacement cylinder", or a
cylinder without a piston seal.
Some folks call any hydraulic cylinder a "ram", but I have never liked that. Depending on
your sources, the term "ram" comes from "hydraulic ram", which is a type of water pump
that utilizes the momentum effect of flow in a pipe. Hydraulic rams long pre-date
fluid power.
Other sources say that a "ram" is a special type of single-acting hydraulic cylinder that
has only one port, the one that extends the rod. It is true that you need no piston
seals in this type of cylinder, and you may not even have a piston at all. (The piston
helps keep the rod in alignment in this case.) This is a "displacement cylinder".
If your single-acting cylinder does have piston seals, but only one port, then you may have
a "plunger cylinder". In this case, the rod is often the same size as the bore, so the
head end always has zero volume. The other type of plunger cylinder would have
piston seals and a smaller rod, but the working fluid would always enter/exit the one
port, with an opening in the rod end to let air enter and exit.
Note that a plunger cylinder extends with the full force of the fluid acting on one side
of the piston, while a displacement cylinder's fluid acts on the full volume of fluid
in the cylinder, on both sides of the piston, yielding less extend force for the same
barrel diameter.
I am prompted to write this after getting one of those emails from our hydraulics guru from
Down Under, Bendan Casey. In it, he equates a "ram" to a "displacement cylinder", or a
cylinder without a piston seal.
Some folks call any hydraulic cylinder a "ram", but I have never liked that. Depending on
your sources, the term "ram" comes from "hydraulic ram", which is a type of water pump
that utilizes the momentum effect of flow in a pipe. Hydraulic rams long pre-date
fluid power.
Other sources say that a "ram" is a special type of single-acting hydraulic cylinder that
has only one port, the one that extends the rod. It is true that you need no piston
seals in this type of cylinder, and you may not even have a piston at all. (The piston
helps keep the rod in alignment in this case.) This is a "displacement cylinder".
If your single-acting cylinder does have piston seals, but only one port, then you may have
a "plunger cylinder". In this case, the rod is often the same size as the bore, so the
head end always has zero volume. The other type of plunger cylinder would have
piston seals and a smaller rod, but the working fluid would always enter/exit the one
port, with an opening in the rod end to let air enter and exit.
Note that a plunger cylinder extends with the full force of the fluid acting on one side
of the piston, while a displacement cylinder's fluid acts on the full volume of fluid
in the cylinder, on both sides of the piston, yielding less extend force for the same
barrel diameter.
