Face it: If you were educated in a four-year mechanical engineering
curriculum, you probably didn't learn much about fluid power. American
universities typically teach fluid mechanics, fluid dynamics, and
thermodynamics. But they seldom delve deeply into the subjects of hydraulics or
pneumatics as a power medium.
That's why most design engineers in this country learn their fluid power on
the job. And it's why they sometimes make mistakes they could easily avoid if
the theoretical underpinnings of the subject were more readily available.
"You may have left school with a mechanical engineering degree, but that does
not mean you have a good grasp of fluid power," notes an engineer from a
major fluid power supply company.
Fortunately, many hydraulic and pneumatic component suppliers offer training
classes for design engineers. Parker Hannifin, Vickers, Mannesmann Rexroth,
Festo and others teach the basics of hydraulics and pneumatics. "Most of these
classes offer 95% of the material that design engineers need," notes Don Caputo,
marketing manager for Parker Hannifin's Hydraulic Valve Division. "We want to
get the word out to customers, many of whom are engineers, who never had this
material in school."
For those who can't immediately attend such programs, Design News
offers a compilation of some of the most common mistakes in fluid power design.
The mistakes cover a broad range, from problems with pneumatic tubing to errors
in hydraulic force calculation. Some may seem elementary. But suppliers agree on
one point: Avoiding them in your projects will not only cut time and save money,
it will eliminate countless headaches.
Hydraulics
Mistake #1: Failure to recognize maximum flow
You're designing a hydraulic actuator that will operate at a prescribed
speed. To accomplish that, you calculate that the cylinder needs a flow rate of
10 gallons per minute (gpm). So you size your hydraulic system for 10 gpm,
right?
Not necessarily. Many applications call for the return stroke to be faster
than the power stroke. In the application described above, it's not uncommon for
the return stroke to need a flow rate of 20 gpm.
If, however, the return lines and filters are sized for 10 gpm, then the user
has a problem. The system builds up heat. Undersized filters fail to properly
clean the hydraulic fluid. Leaks result.
Remedy: Consider the return rate
Remember, the flow rate needed for cylinder extension is not always the
system's fastest flow rate. "Typically, the user wants to extend the cylinder
with a great deal of control," Caputo says. "But on the way back, they're not
doing any work, so they want the cylinder to get back as fast as it can." In
most applications, Caputo says, the return rate is twice that of the extension
flow rate.
Mistake #2: Undersized piping
After selecting a hydraulic motor, engineers often look at the motor's port
sizes before choosing their piping. If the motor's port sizes are, say,
three-quarters of an inch, then they choose three-quarter-inch fittings and
three-quarter-inch outer diameter piping.
That's a mistake. Too often, such snap decisions result in undersized piping.
And undersized piping, in turn, causes larger pressure drops and more heat
generation than the system is designed for.
Remedy: Check ID, flow velocity, pressure drop
A designer who quickly sizes piping simply by looking at the motor's port
risks making several mistakes. First, a three-quarter-inch outer diameter
is too small. Why? Because a three-quarter-inch OD may have an inner diameter of
as little as half an inch. Second, selection of piping requires more thoughtful
consideration. Designers must consider velocity of flow through the pipe and
pressure drop per foot of pipe (see equations above and below). Experts say that
pressure drop can be important, especially in systems using long sections of
piping. They also recommend that designers maintain flow velocities with certain
parameters. Exceeding those parameters may cause turbulence in the flow, which
can affect the Reynolds number of the fluid.
Mistake #3: Reservoir size too small
By dissipating heat, reservoirs in any hydraulic system play an important
role. If the reservoir is undersized, however, it can't properly dissipate the
heat. As a result, performance suffers and components wear out earlier. "A lot
of users assume their reservoir is big enough without sitting down to calculate
the heat dissipation," Caputo says. "If they would do some simple calculations,
they might find that their system needs a bigger reservoir or even a heat
exchanger."
Remedy: Calculate dissipation capabilities of the reservoir in
horsepower In general, a 60-gallon tank will dissipate half a
horsepower, given a 50° temperature differential between the oil in the
reservoir and the air outside it. Compare that to your system's horsepower. Use
a bigger reservoir or a heat exchanger, if necessary, depending on your
packaging constraints.
Mistake #4: Proportional valves too big
When selecting a directional valve, designers usually check their system's
flow rate, open a catalog, and look for a valve with a corresponding rating.
So what's wrong with that? Nothing--if you're picking a directional valve.
Proportional valves, however, are different. "Proportional and servo valves
exhibit their control through a high pressure drop," notes Larry Schrader,
director of motion control training for Parker Hannifin. The valve's flow
rating, he says, is usually based on a specific pressure drop. Therefore, if
your application's pressure drop is significantly different than the rated
pressure drop of the valve, you'll probably select the wrong valve. Usually,
Schrader says, engineers end up with an oversized proportional valve.
If they do select an oversized valve, users are unlikely to get true
proportional performance. In most cases, the valve will open all the way before
it's supposed to, denying users the resolution that they seek.
Remedy: Use pressure drop to determine flow rate
Experts recommend that designers use this method to check the flow rating of
their valve. In most cases, they say, the flow rating they obtain by this method
will differ from the flow ratings in the catalog.
Mistake #5: Specific gravityproblems
In many cases, users want to replace conventional hydraulic oil with
phosphate esters of water glycol fluids. That's fine, say experts, as long as
you understand that hydraulic pumps can't lift those fluids as easily as they
lift hydraulic oil. The reason: Conventional hydraulic oil has a specific
gravity of about 0.85, while water glycol typically is about 1.0. Phosphate
ester is even heavier at 1.1.
If design engineers don't make special accommodations for those heavier
fluids, users soon notice that pumps make too much noise. Ultimately, cavitation
of the pump occurs.
Remedy: Put the reservoir higher than the pump
Manufacturers call this "flooded suction." The pump doesn't need to work as
hard to move the heavier fluid. If you're not sure about the specific gravity of
the fluid you're using, call the pump's manufacturer to see if they require a
flooded suction for that fluid.
Pneumatics
Mistake #6: Force miscalculation
If you know the loads, sizing a cylinder is easy. Unfortunately, knowing the
loads can sometimes be difficult. Forces caused by friction and acceleration are
more difficult to calculate, and are often overlooked. Worse, designers can't
always foresee how the machine will be used. "For the design engineer, it's not
always as easy as, 'I need to lift 50 pounds,'" says Jerry Scherzinger, senior
support engineer for Bimba Manufacturing. "A lot of times the designer is coming
up with a best estimate, and there are many factors involved in estimating that
force." Too often, Scherzinger says, the designer underestimates the force
requirement, rather than overestimates it. As a result, the system doesn't move
the load fast enough, or doesn't move it at all.
Remedy: Oversize by 25%
That's what Scherzinger recommends to account for frictional loads. If you
suspect that the cylinder will be subjected to greater loads later on, you might
even consider exceeding that 25% rule of thumb. The only downside to doing that
is cost: Initial costs and operating costs can rise. In the long run, however,
users usually find that a larger cylinder provides greater benefits. "Don't
hesitate to oversize the cylinder," he says. "The advantages of oversizing
outweigh the disadvantages."
Mistake #7: Wrong valve for E-stop
Emergency-stop situations demand that a pneumatic system come to a complete
halt. Sometimes, however, a pneumatic valve is already shifted when power is cut
off. Afterwards, the valve shifts back and the associated pneumatic cylinder
moves. That scenario can take place when certain five-way valves are used with
double-acting cylinders. And the results, in some cases, can be damaging or even
dangerous for personnel.
Remedy: Check with valve manufacturer first
In most cases, users notice the problem before damage occurs. By that time,
however, the valve has already been specified and installed. As a result,
changing to the right valve can be time-consuming and costly. For that reason,
experts say that engineers must be vigilant early in the design process. "They
have to understand their machine and ask all of the 'what if?' questions," notes
Daniel Sandoval, didactic training manager for Festo. "What if power is cut-off?
What if the PLC goes bad? What if someone hits the E-stop? The solutions to
these problems have to be dealt with in hardware. It's not permissible to remedy
those problems in software." Because there are many different types and brands
of valves, Sandoval says designers must consult with the valve manufacturer to
learn how the valve will behave when power is cut off.
Mistake #8: High-speed undersizing
High-velocity pneumatic applications require sufficient force and high flow
rates. Unfortunately, system designers don't always provide both of those
features. Too often, air cylinders are too small to provide the force needed for
quick acceleration. And valves don't have the necessary flow rates to achieve
high speed.
Remedy: Double the bore diameter of the chosen cylinder
For speeds above 16 inches/second, if you're planning on selecting a one-inch
cylinder, select a two-inch model instead. Also, pneumatics manufacturers say
that designers must look carefully at the flow ratings of the valves they plan
to use in high speed applications. Catalog ratings are usually sufficient, they
say, but are too often ignored or misinterpreted.
Mistake #9: Failure to specify tough-enough construction
Pneumatic-component manufacturers say they see them all the time: cylinders,
working in tough, corrosive environments, unable to stand up to every day
wear-and-tear. Before long, the cylinders draw in dirt, then fail to perform
properly. In most cases, they say, the problem occurs because designers never
foresee the breadth of application for their technology. "Sometimes a designer
will design a system that's so good, three or four industries can use it,"
Scherzinger says. "All of a sudden, their equipment is in an application where
they never expected it."
Remedy: Better materials
If you suspect that your machine will be used in such applications as food or
pharmaceutical processing, be prepared to improve the materials of construction
of your cylinders. Stainless steel or anodized aluminum bodies resist corrosion
more effectively, particularly in wash down environments. Also consider use of
plastic end caps, wipers, scrapers or rod boots as a means of preventing ingress
of dirt into the cylinder.
Mistake #10: Insufficient tubing
Most designers are trained to think of a pneumatic system as a collection of
pumps, cylinders, and valves. Somewhere in between those pumps, cylinders, and
valves, however, lies an equally important component: tubing. Improperly
designed tubing can cause disturbances in flow and pressure. Too often, it
becomes the limiting factor in a pneumatic system.
Remedy: Pay attention
Experts say that tubing mistakes fall into two categories: too long or too
skinny. Long sections of tubing create unwanted pressure drops and skinny tubes
cause valves to work improperly. Unfortunately, they say, designers can only
remind themselves to be aware of the problems caused by improperly designed
tubing. "When you look at a drawing, the tubing looks like a bunch of
insignificant lines," Sandoval says. "But if you don't pay attention to it, it
can cause a lot of trouble for you."
Determining pressure drop in a length of pipe
P= µ Q
Where P=pressure drop/ft of pipe
µ=viscosity (ssv)
Q=flow
d=ID of pipe
Determining velocity of flow through a pipe
V= 0.3208(GPM)
Where V =flow velocity
GPM =gallons per minute of flow into the system
A =area inside the pipe (in2)
Determining heat dissipation of a reservoir
HP=0.001 A³T
Where HP =horsepower dissipated
A =surface area of reservoir (ft2)
³T =temperature difference between oil in tank and air outside (deg F)
(1HP=2544 BTU/hr)
Sizing a proportional valve by flow rate
QR=QOUT -------
Where QR =valve's rated flow for your application
QOUT =output flow needed for application
³PR =rated pressure drop of proportional valve
³PA =actual pressure drop needed for application
Back-to-school
Many companies offer training for engineers who want to learn the theory
behind fluid power design. Following are four companies--three hydraulic and one
pneumatic--that offer training in these areas.
Eaton Hydraulics 612-937-7132
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