If you're hanging pictures or putting together a bookcase,
a hammer is a perfect tool: simple, cheap and entirely
effective. But if you're building a two-story house,
installing hardwood floors or running your own furniture
repair shop, you may want to spring for a nail gun.
These powerful machines launch nails at high speed, fully
embedding them in a piece of wood in only a fraction of a
second. Obviously, such a machine can save you hours of toil
and sweat. They take almost all of the work out of nailing.
Photo courtesy Hitachi
Power Tools There are a
wide range of nail guns on the market today. Different
designs are configured for specific nail sizes and
applications.
In this edition of HowStuffWorks,
we'll find out how these popular machines launch nails at such
astounding speeds. As it turns out, there are a wide variety
of nail guns on the market, employing a range of physical
principles.
Spring-loaded Design At its most basic
level, a nail gun has only two jobs:
It needs to concentrate a great deal of hammering
force into a single mechanized blow, which can be repeated
rapidly.
It needs to load a new nail after the previous
nail is ejected.
There are any number of machines
that could handle these tasks. Since we can't look at every
single model, we'll investigate a few representative designs.
The simplest nail guns use ordinary springs to
generate the hammering force. You can see how this sort of gun
works in the animation below:
In this design, the motor
(powered by a battery or
household
AC current) rotates two drive axles. The front axle moves
a small scooper plate (a), a metal disc with a curved
groove cut into it, and the rear axle moves a gear train,
which turns a small triangular metal cam (b). Here's what
happens when when you pull the trigger:
The triangular cam turns, pushing down one end of a
lever (c).
The lever pivots, pushing up on the hammer (d). As the
hammer pushes up, it compresses two springs (e).
The hammer has a small knob (f) attached to one end. As
the lever pushes the hammer up, the turning scooper plate
catches hold of this knob. At the same time, the rotating
cam (b) releases the lever, which releases the hammer. The
hammer is now held in place by the scooper plate.
As the scooper plate turns, it lifts the hammer higher
and finally releases it.
The two compressed springs drive the hammer downward at
high speed. If a nail is in position, the hammer launches it
from the gun.
The gun in this diagram uses the most popular sort of
nail-loading mechanism. The nails are glued together
loosely, in a long strip. This nail strip loads into the gun's
magazine, which feeds into the "barrel" of the gun. Springs in
the base of the magazine push the nail strip into the barrel.
When the hammer comes down, it separates the first nail from
the strip, driving it out of the gun and into the wood. When
the hammer is cocked back, the springs push the next nail into
position.
Photo courtesy Hitachi
Power Tools A standard
nail gun magazine: A spring pushes the nails up into the
feed mechanism, which sets in up in front of the
blade.
One advantage of this system is that the glue helps
secure the nails. When the nail is hammered into the wood, the
intense friction heats the glue to the melting point. Once the
nail is in place, the glue quickly hardens again, fusing the
nail to the surrounding wood.
In this gun, the electric motor only cocks the gun; the
coiled springs do the real hammering work. In the next
section, we'll look at a gun design that uses electric power
to drive the nails directly.
Electromagnetic Design One effective
hammering device is a solenoid. A solenoid is a simple
sort of electromagnet
used in a variety of machines.
If you've read How
Electromagnets Work, then you know the basic idea behind
electromagnetic devices: Running electricity through a wire
generates a magnetic field. You can amplify this magnetic
field by winding the charged wire in a coil. Just like a
permanent magnet, an electromagnetic field has a polar
orientation -- a "north" end and a "south" end. If you put two
magnets together, the north ends repel one another, as do the
south ends, but the north and south ends are attracted to each
other.
In an electromagnet, you can alter the orientation of the
poles. If you reverse the flow of the current, the north and
south ends of the electromagnet switch places.
A solenoid is an electromagnetic coil with a sliding
piston inside it. In a nail gun, the piston is made of
magnetic material. When you apply current one way through the
coil, the electromagnetic field repels the magnetic piston,
pushing it out. But when you reverse the current, the polar
orientation switches and the electromagnet draws the piston
back in. Some solenoids have a spring mechanism to draw the
piston back in.
An electromagnetic nail gun uses such a solenoid as a
hammer. When you pull the trigger:
The electrical circuit runs the current through the
electromagnet so that the piston extends downward.
Typically, the piston is attached to a sturdy blade.
The blade makes contact with the nail, forcing it out of
the gun.
At the bottom of the cylinder, the piston hits an
electrical switch.
Throwing this switch reverses the electrical current
running through the electromagnet. The electromagnet draws
the piston back in for another hit.
Solenoids are effective and reliable, but they are somewhat
limited in power output. A solenoid gun may not be able to
drive a nail through tougher substances, at least not in a
single blow. In the next section, we'll look at a more
powerful type of nail gun that has dominated the market.
Pneumatic Design The most popular sort of
nail gun is the pneumatic nailer. In these machines,
the hammering force comes from compressed air,
typically generated by a separate gas-powered air compressor.
A standard air compressor works on the same principle as a
water pump. It has one or more piston cylinders, which draw
air in from the atmosphere on the upstroke and push it out to
the gun on the downstroke. This generates a constant supply of
compressed air, which flows through a hose into the gun's
air reservoir.
A pneumatic nailer uses the same sort of hammer as a
solenoid nailer: It has a sliding piston that drives a long
blade. When the air pressure above the piston head is greater
than below it, the piston is forced downward. When the air
pressure below the piston is greater than above it, the piston
stays up. The trigger mechanism serves to channel the flow of
compressed air to shift this balance. The diagram below shows
a typical valve system used in this type of gun:
In this design, a movable valve plunger (a) sits
above the piston head (b). When the trigger is in the released
position, compressed air can flow to both sides of the valve
plunger. It flows directly through the air reservoir to a lip
around the bottom of the plunger, through the trigger valve
(c) and a small plastic tube (d) until it reaches the area
above the plunger. Since compressed air flows to both sides of
the plunger, air pressure balances out. But the plunger is
also attached to a spring (e), which pushes it downward. This
shifts the pressure balance: When the trigger is released,
there is always greater pressure above the plunger than below
it.
This imbalance keeps the plunger pressed against the
seal surrounding the piston head. With the plunger in this
position, the compressed air flowing into the gun can't reach
the top of the piston to push it down.
Here's what happens when you pull the trigger on this
pneumatic nail gun:
The trigger valve closes and opens a passageway to the
atmosphere. With the trigger valve in this position, the
compressed air can't flow to the area above the valve
plunger.
There is greater pressure below the plunger than above
it. The plunger rises up, and the compressed air makes its
way to the piston head.
The compressed air drives the piston and the blade
downward, propelling the nail out of the chamber.
As the piston slides downward, it drives the air inside
the cylinder through a series of holes, into a return air
chamber (f).
As more air is pushed into the
chamber, the pressure level rises. When you release the
trigger, compressed air pushes the plunger back into place,
blocking the air flow to the piston head. With no downward
pressure, the compressed air in the return air chamber can
push the piston head back up. The air above the piston head is
forced out of the gun, into the atmosphere.
Pneumatic nail guns work very well, even at driving thick
nails through hard material. But they are somewhat cumbersome
tools: You need to drag around a bulky air compressor to power
one. In the next section, we'll look at a high-powered nail
gun design that doesn't need an external power source.
Combustion Design One of the newest nail gun
machines to hit the market is the combustion nailer.
These portable guns generate hammering power with internal
combustion, the same force that keeps your car going (see
How Car
Engines Work).
At the most basic level, combustion guns are a lot like
pneumatic nailers. They have a long blade attached to a
sliding piston, which is moved by an imbalance in atmospheric
pressure. The piston moves downward when there is greater
pressure above it, and it moves upward when there is greater
pressure below it.
The difference between pneumatics and combustion models is
the source of the pressure imbalance. Just like your car,
combustion guns have a reservoir filled with a flammable
gas. An electronic control mechanism releases a little of
this gas into a combustion chamber just above the
piston head. A small fan in the combustion chamber
vaporizes the gas, mixing it up with the air particles.
This design has a double trigger mechanism. To hammer a
nail, you need to pull down the trigger and press the barrel
up against the surface at the same time. Pressing the barrel
down pushes back a metal valve around the main cylinder. This
controls the gas intake and exhaust cycle of the gun. Here's
what happens when you activate both triggers:
The valve piece (shown in green) slides back around the
cylinder. As it moves back, the valve closes the exhaust
port at the top of the gun. As it keeps moving back, it
briefly opens the intake port from the gas supply (shown in
blue), and then closes it again. This lets a small amount of
gas inside, which the fan mixes with the air.
The gun's battery sends a charge to a spark
plug at the top of the combustion chamber. This ignites
the gas, causing a small explosion.
The pressure of the explosion propels the piston
downward, driving the blade into the nail so that the nail
is shot from the gun. As the piston pushes downward, it
compresses the air in the cylinder.
When you lift the gun off the nailing surface, the valve
slides back down. This opens the exhaust port, so the
exhaust can escape.
The compressed air in the return chamber pushes the
piston back up, to the "ready" position. Essentially, the
air in the return chamber acts like a spring.
The second trigger in this gun is intended as a safety
device: you have to pull the trigger and press the gun against
a surface to shoot a nail. As we'll see in the next section,
this mechanism helps prevent some nail gun injuries, but it
may lead to other sorts of accidents.
Safety Measures In the last section, we saw
that a combustion nail gun has two separate firing triggers.
Most modern nail guns are built with similar safety
catch devices, to keep people from accidentally shooting
nails through the air. In the diagram below, you can see how a
simple safety mechanism might work in an electric nail gun.
The gun has a catch that holds the blade in place. To release
the catch, you have to press the gun against something.
This sort of device may reduce the risk of certain
injuries, but it is not a foolproof system. With this type of
gun, carpenters tend to keep the main trigger depressed at all
times, using the safety catch as the actual triggering
mechanism: When they press the gun down on a surface, it
automatically drives a nail in.
When you use the gun this way, it's a lot easier to
accidentally fire a nail. All you have to do is press the gun
barrel against your leg or bump into someone with it. Every
year, hundreds of people are rushed to the emergency room with
these sorts of injuries. To reduce this risk, some
manufacturers build a sequential trip system into their
guns. In this type of nailer, you have to release and depress
the trigger every time you want to fire a nail.
Photo courtesy Hitachi
Power Tools This nailer
uses a coil-type nail magazine. The connected nails are
arranged in a long belt, which winds around a
spool.
No matter how a gun is designed, it should be appreciated
for what it is: an extremely powerful, dangerous machine. Just
like a handgun, power nailers fire projectiles at high speed
-- some designs launch nails at speeds reaching 1,400 feet
per second (427 meters per second).
In this case, convenience definitely comes at a price.
Fumbling with a power nail gun might put you intensive care,
while the worst you can expect from your old hammer is a
bruised thumb.
For more information on nail guns and related topics, check
out the links on the next page.
