Gravity knife

Gravity knife

A gravity knife is a knife which opens by the forces of inertia or gravity.cite book | last = Langston | first = Richard | title = Collector's Guide to Switchblade Knives: An Illustrated Historical and Price Reference | publisher = Paladin Press | date = 2001 | location = Boulder, Colorado | pages = 224 | isbn = 1581602839 ] Similar to a switchblade, they are made as side folding blades and OTF blades (out-the-front or telescoping). However, instead of being spring driven, the knife needs to be manually "flipped" out of the handle. Military gravity knives will lock open. Some English and civilian gravity knives do not mechanically lock open but rely on a strong friction cam surface against the blade.cite book | last = Flook | first = Ron | title = British and Commonwealth Military Knives | publisher = Howell Press Inc. | date = 1999 | pages = 256 | isbn = 1574270923 ] Factory made gravity knives have various types of buttons, triggers or fulcrum levers, but are usually dual function to release the blade from both the open and closed positions.


One of the most recognizable Gravity knives is the World War II Fallschirmjager-messer German military paratrooper model OTF that locked open in case the user had to cut himself free from a parachute that had become tangled in a tree or other obstruction.cite web| title = AKTI APPROVED KNIFE DEFINITIONS | publisher = American Knife and Tool Institute | date = 2005 | volume = 7 | issue = 2|url = ] This is a true gravity knife that requires the user to point the knife down to let gravity draw out the blade, it 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 bakelite 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 stilettos 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 switchblade and the butterfly knife (or balisong), the sliding knife and occasionally common folding knives (see penny knife).cite web| last=Muldoon| first=Gary|title = Is Butterfly Knife a Gravity Knife? | publisher = Conflict Defender Office, Monroe County, New York | date = 2006 | volume = 5 | issue = 4|url = ] 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 second 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 sear 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".Fact|date=July 2008 Legal definitions say this as an illegal inertia knife because there is no spring.Fact|date=July 2008 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.Harvard reference | Surname=Dick | Given=Steven| Title=The Working Folding Knife | Year=1997| publisher= Stoeger Publishing Company |pages = 280|ISBN= 9780883172100 ] 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.Fact|date=July 2008 Around 1965, New England local laws prohibited bayonet points with a sharp upper.Fact|date=July 2008 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.Fact|date=July 2008


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 locks; 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.cite news| last=Szilaski |first= Joe| title = State Court Says LinerLocks are Gravity Knives| publisher = Blade Magazine | date = 2008 | volume = 32 | issue = 1 ] However, it has been ruled in New York State that butterfly knives are not classified as gravity knives any longer. ["People v Zuniga", 303 AD2d 773, 759 NYS2d 86 (2d Dept 2003)] ["People v Dolson", 142 Misc2d 779, 538 NYS2d 393 (County Court 1989)] ["People v Mott", 137 Misc2d 757, 522 NYS2d 429 (County Ct 1987)] Still other city statutes refer to the terms "primarily a weapon with no useful purpose".

Assisted Opening Knives 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.Fact|date=July 2008 Many knife collectors sarcastically ask, does term inertia knife term apply only if you hold the handle when opening?Fact|date=July 2008 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.Nappen, Evan(2003), "Are Switchblades Sporting Knives", Sporting Knives 2003: 60-65, ISBN 0-87349-430-X]


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.Fact|date=July 2008 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.Fact|date=August 2008

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.

See also

*Pantographic knife


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