A
gravity knife is a
knifeA knife is any cutting edge or blade, handheld or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools. Originally made of rock, flint, and obsidian; knives have evolved in construction as technology has with blades...
that opens by the force of inertia or gravity. Similar to a
switchbladeA switchblade is a type of knife with a folding or sliding blade contained in the handle which is opened by a spring when a button or lever on the grip is pressed....
, it is made with a side-folding blade or an out-the-front (OTF), or telescoping, blade. The gravity knife is different from the switchblade, which is
springA spring is an elastic object used to store mechanical energy. Springs are usually made out of hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
-driven, in that it needs to be "flipped" out of the handle manually. Military gravity knives lock open. Some English and civilian gravity knives do not mechanically lock open but rely on a strong friction cam surface against the blade.
A
gravity knife is a
knifeA knife is any cutting edge or blade, handheld or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools. Originally made of rock, flint, and obsidian; knives have evolved in construction as technology has with blades...
that opens by the force of inertia or gravity. Similar to a
switchbladeA switchblade is a type of knife with a folding or sliding blade contained in the handle which is opened by a spring when a button or lever on the grip is pressed....
, it is made with a side-folding blade or an out-the-front (OTF), or telescoping, blade. The gravity knife is different from the switchblade, which is
springA spring is an elastic object used to store mechanical energy. Springs are usually made out of hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
-driven, in that it needs to be "flipped" out of the handle manually. Military gravity knives lock open. Some English and civilian gravity knives do not mechanically lock open but rely on a strong friction cam surface against the blade. Factory-made gravity knives have various types of buttons, triggers, and fulcrum levers, which usually are used to release the blade from both the open and the closed positions.
Designs
One of the most recognizable gravity knives is the
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
Fallschirmjäger-Messer German military
paratrooperare German paratroopers. Fallschirmjäger of Germany in World War II were the first to be committed in large-scale airborne operations. They came to be known as the "Green Devils" by the Allied forces they fought against...
model OTF, which locked open in case the user had to cut himself free from a tangled parachute. This is truly a gravity knife: it requires the user to point the knife down to let gravity draw out the blade; this can be assisted with a fencing salute movement. This model had a folding rigging spike in addition to the primary blade.
The military issued German gravity knife has evolved since the 1940s, with size and minor mechanical details. The
bakeliteBakelite , or polyoxybenzylmethylenglycolanhydride, is an early plastic. It is a thermosetting phenol formaldehyde resin, formed from an elimination reaction of phenol with formaldehyde, usually with a wood flour filler. It was developed in 1907–1909 by Belgian chemist Dr...
handle trap door model, without spike is also produced in a civilian version with an all metal handle named the "silk ripper". The civilian designed OTF gravity knives are of such small size they are not considered as weapons for military use, but are still considered to be restricted knives.
Some knives that will open by gravity, were not intended or advertised by the manufacturer as such. Knives that will lock open, but do not have any closed position lock device are referred to as "False" Gravity blades. Knives such as a modified linerlock, will not lock a blade in both positions. Other false gravity knives or "flick knives" are folding
stilettoA stiletto is a short knife or dagger with a long slender blade of various designs. This blade is primarily a stabbing weapon: its narrow shape, ending in a rigid pointed end, allows it to penetrate deeply. It is not suited for cutting, even with edged examples...
s made early in the 1960s. Immediately after switchblade prohibition in the U.S.A. these early folding stilettos did not have the pronounced blade heel (internal surface) in the pivot area allowing the longer 11 and 13 inch models to easily flick open. Modern (post 1965) folding stilettos have blade heels that are intentionally pointed and ratchet against the lockback mechanism preventing inertia opening. Other knives commonly mistaken for gravity knives include the automatic OTF knife, the
switchbladeA switchblade is a type of knife with a folding or sliding blade contained in the handle which is opened by a spring when a button or lever on the grip is pressed....
and the
butterfly knifeA balisong, otherwise known as a butterfly knife or a Batangas knife or sometimes called Bente Nueve, is a folding pocket knife with two handles counter-rotating around the tang such that, when closed, the blade is concealed within grooves in the handles. In the hands of a trained user, the knife...
(or balisong), the
sliding knifeAn OTF Knife, also known as an out-the-front knife, sliding knife, or telescoping knife, is a pocketknife with a blade that opens and closes through a hole in one end of the handle...
and occasionally common folding knives (see
penny knifeThe penny knife is the most basic kind of folding knife, where the blade folds in and out of the handle freely without a spring or other locking device to hold it in position...
). In the latter part of the 19th Century, the Eagle Pencil company produced a small tube-type knife held in place by a pair of grabbers that open and closed by means of a button that was designed to be used as an eraser to scrape mistakes off of a letter.
Mechanical design
The illustration at right, is of a third generation OTF German Fallschirmjäger-messer. This model has a bakelite handle and side mounted trap-door dust cover. Next is a cross section of the trigger and sear hook, shown behind the blade tang, locking the blade out. The blade will have two pins, or pressed ball bearings that have a tight running fit in the groove within the handle track. These pins keep the blade aligned, and also being larger than the opening, prevent the blade from falling out of the handle. Some models have a blade notch at the point for the
searIn a firearm, the sear is the part of the trigger mechanism which holds the hammer or striker back until the correct amount of pressure has been applied to the trigger; at which point the hammer or striker is released to discharge the weapon...
to engage the blade when closed. On this model, the sear hook spring pressure is enough to keep the blade in.
This illustration shows the internal parts of a 1960s Japan import folding knife.Collectors classify it as a "false gravity knife". Legal definitions include this inertia knife with illegal switchblades even though it is not spring activated. The knife will lock open but will not lock closed. The toggle lever was designed to unlock to close the blade with a cam surface lifting the pin in the lockback bar. But with user manipulation, the toggle will also lift the lockback allowing the blade to swing freely, and can be flicked open.
Some gravity knives are called "Conversions" or knives with after-market modifications. The knife at the right does not lock closed, and is a false gravity knife. It can still be opened manually without touching the toggle switch. The addition
of the toggle, which is attached to an internal bellcrank-arm, will lift the
lockback mechanism to allow the blade to swing easier. This knife mimics a rare device from the 1960s, thus allowing collectors to acquire an inexpensive facsimile.
Collectors have observed many trends in knife design that are resultant from legal rulings. Around 1965, New England local laws prohibited bayonet points with a sharp upper. A dealer ordered stilettos with clip-point blades starting a trend, so even though gravity knives were already outlawed, they also became available with clip points.
Legality
While collectors use the term Gravity Knife (as a particular mechanism) to describe specific styles of knives that lock open and closed within a singular handle, excluding butterfly or balisong knives, assisted opening and/or
liner locksThe Walker Linerlock is a locking system developed by custom knifemaker Michael Walker in 1980 for use on folding knives.When the blade of the knife is in the opened position, it is held in place by a leaf spring that butts up against the tang of the blade to prevent the blade from closing...
; police officers and lawyers may use the term Gravity Knife for the general category of easy opening one handed knives without switchblade main springs.
In many jurisdictions a larger category of knives fall under the definition of gravity knives or switchblades, either explicitly or through any easily opening blade or use of a locking mechanism, because of a 1950s New York City law stating "any knife that is caused to open by means of button, lever, trip spring or switch, gravity or inertia opening and centrifugal force". Recent New York rulings have determined that liner locks may be considered gravity knives. However, it has been ruled in New York State that butterfly knives are not classified as gravity knives any longer. Still other city statutes refer to the terms "primarily a weapon with no useful purpose".
Assisted Opening KnivesAn assisted-opening knife is a type of knife which uses an internal mechanism to open the blade from a closed position when slight pressure is applied to the thumbstud or the flipper....
contain a spring blade that only cycles once the blade is pushed partially open with a thumb stud, past an internal "ball detent". This type of knife is very fast opening. But, since the knife appears to resist gravity or inertia, and requires thumb pressure to open manually, they seem temporarily exempt from "gravity" classification. Thumb studs were not mentioned in 1950's statutes, laws are not recently updated, court rulings are not widely publicized. Another name for this type of knife is a "Torsion Bar" knife, named after the internal mechanism, the bias for the folding blade to remain closed is limited to around 30 degrees, then the torsion bar will flex and the bias is shifted to swing the blade open.
Legal terminology battles over the method of opening has little to do with a knife's speed and lethality. Many knife collectors sarcastically ask, does the term "inertia knife" apply only if you hold the handle when opening? Since a great majority of regular folding knives can be inertia opened by grasping the blade and using the handle weight to overcome any bias to remain closed; opening this way results in a knife held by the blade ready for throwing, or the knife can be readily flipped to a fencing grip.
Alternatives
Since the laws always begin with "be caused to OPEN by means of . . ." there are a few models of knives built to ridicule the wording. Some knives have springs set up in reverse, to cause the knife to snap shut, or bias closed, after manually opening. Removal of the spring will create a gravity knife.
Given that a large number of knives originated since 1970 are designed to be easily opened one-handed (often by means of a thumb-hole or thumb-stud), the practical advantages of the gravity knife have been rendered obsolete.
Paratrooper Knife
In addition to the German gravity knife shown above, the term Paratrooper Knife may also refer to an automatic switchblade clip point jack-knife with optional folding cutting hook.