Impact wrench

How it works

A motor, driven by compressed air, mains electricity or a battery, turns a hammer. The hammer is connected to the motor by a spring or a clutch-and-cam arrangement that stores energy, and it strikes an anvil that carries the square drive. Pott’s 1935 patent states the object as a tool producing “a succession of impacts of great force” by momentarily accumulating energy to be released when the hammer strikes the driven part. Fitch’s 1940 patent for Ingersoll-Rand adds that the work is subjected to a series of impacts when the torsional force of the tool proves ineffective.

The effect on the operator is the reason the tool exists. The 1951 court opinion in Ingersoll-Rand v. Black & Decker states that the general purpose of an impact wrench is to lessen the strain on the operator caused by recoil when a nut seats, and that the design also allows a much lighter tool with power equal to a heavier one. A 1939 patent by Shaff describes the earlier problem: pneumatic nut runners with a rigid spindle stalled at the end of each operation and produced a noticeable recoil that the operator had to hold against.

Tightening torque depends on how long the tool impacts. The Milwaukee manual states that the longer a bolt or nut is impacted the tighter it becomes, and DEWALT’s manual lists voltage, socket size, bolt size, thread condition, material and fastening time as factors that change the torque delivered.

Anatomy

Motor: Air motor, universal electric motor or battery-powered motor that turns the hammer.
Hammer and spring (impact mechanism): Pott’s patent describes a hammer joined to the motor through springs that store energy and release it as blows on the driven member. Later designs, such as Fitch’s, use clutch members and cams between hammer and anvil.
Anvil with square drive: The output member struck by the hammer. In the DEWALT manual a socket is fitted by aligning its hole with the detent pin on the anvil and pressing it on.
Detent pin: A spring pin that holds the socket on the square drive; pressing it through the hole in the socket with a small pointed object releases the socket.
Forward/reverse switch: On the DEWALT corded models the bottom of the rocker switch runs the tool forward and the top reverses it.
Pistol-grip housing: Ingersoll-Rand’s catalogs of 1940 and 1959 show and describe pistol-grip Impactool air and electric models in several drive sizes.

Variants

Pneumatic (air) impact wrench: Driven by compressed air through a hose. Ingersoll-Rand’s 1940 catalog lists numbered sizes with a table of working speeds at 90 psi.
Electric (corded) impact wrench: Ingersoll-Rand’s 1959 catalog lists electric Impactools in several drive sizes up to 1 inch. DEWALT’s manual covers corded 1/2 inch and 3/4 inch models.
Cordless impact wrench: Battery-powered, as in the Milwaukee M18 manual, which covers three square-drive models and one hex-drive impact driver.
Impact driver: The same mechanism with a hex bit holder in place of the square drive; see the impact-driver entry.

Specifications

Drive size · in: 1/2 and 3/4 in the DEWALT corded manuals; Ingersoll-Rand’s 1959 catalog lists drive sizes up to 1 inSockets must match the square drive.
Output torque · ft-lb: 325 (1/2 in models) and 434 (3/4 in model) in the DEWALT corded manualsThe manuals say actual fastening torque varies with voltage, socket, bolt, thread condition, material and time.

Made to

IEC 62841-2-2
ASME B107.4
ISO 1174-1
OSHA 29-CFR-1926-302

Safety and care

Safety

DEWALT and Milwaukee both state that only impact sockets and accessories may be used. Non-impact sockets can shatter or break and cause injury, and sockets should be inspected for cracks before use (DEWALT).
DEWALT warns that the fastener and the joint must withstand the torque the tool can produce, since excessive torque may cause breakage and personal injury.
DEWALT’s manual warns that bits, sockets and tools get hot during operation and that gloves should be worn to touch them.
OSHA 29 CFR 1926.302(b)(2) requires safety clips or retainers on pneumatic impact tools to prevent attachments from being accidentally expelled, and (b)(1) requires pneumatic tools to be secured to the hose or whip by a positive means.

Care

DEWALT states that its corded impact wrench was lubricated at the factory and should go to a certified service center for cleaning and inspection every two to six months depending on use, with more frequent service for production or heavy-duty use or heat exposure.
The DEWALT manual says tools out of service for long periods should be relubricated before being put back into service.

Tips

Let impact time set the torque, and check it with a torque wrench
Milwaukee’s manual says the longer a fastener is impacted the tighter it gets. It advises practicing on various fasteners while noting how long it takes to reach the desired torque, checking with a hand torque wrench, and adjusting the impact time up or down.1
Treat the tool’s torque as a variable
DEWALT lists low voltage from a generator or long extension cord, the wrong socket size, bolt size and grade, rust or debris on the threads, material and surface finish, and fastening time as factors that change the torque delivered, and says to always check torque with a torque wrench.1
Expect to loosen at about three-quarters of the tightening torque
Milwaukee’s manual says the torque needed to loosen a fastener averages 75% to 80% of the tightening torque, depending on the condition of the contacting surfaces.1
Rock a frozen fastener with the reverse switch
DEWALT says reversing the motor allows “rocking” a fastener to break it loose.1
Run gasket joints down light and finish by hand
For light gasket jobs Milwaukee’s manual says to run each fastener down to a relatively light torque and use a hand torque wrench for the final tightening.1
Be careful with small fasteners
Milwaukee’s manual says small fasteners need less impacting to reach their optimum torque, so avoid excessive impacting to prevent damaging fasteners or workpieces.1
Release the socket through its detent hole
A socket is held by a detent pin in the anvil. DEWALT instructs the user to align the hole in the socket with the pin to fit it and to press the pin in through the hole with a small pointed object to remove it.1

History

Earliest documented

30 January 1932 (Pott application filed); patent granted 27 August 1935

Evansville, Indiana, United States

US Patent 2,012,916, “Impact Tool,” by Robert H. Pott of Evansville, Indiana, describes a motor-driven hammer joined by helical springs that store energy and release it as impacts on a driven tool head, for “applying a torsional force.” The 1951 court opinion in Ingersoll-Rand v. Black & Decker says Pott illustrated one use of his mechanism as an automatic wrench, and that Pott and a partner, Holsclaw, built 24 or 25 wrenches as described in the patent, which did not achieve commercial success.1 2

The impact wrench exists to deal with reaction. A rigid-spindle pneumatic nut runner of the 1930s kept turning against the nut until the air motor stalled, and Shaff’s 1939 patent describes the sudden halt as a noticeable recoil that the operator had to hold against. The district court in 1951 gave the same account of the purpose of the impact wrench: it lessens the strain on the operator when the nut seats and permits a lighter tool with the power of a heavier one. Ingersoll-Rand’s 1940 catalog lists the same benefits to buyers, stating little torque reaction on the operator, lighter weight, and removal of frozen nuts that a torque-type wrench cannot start.

The mechanism comes from Pott’s patents, filed in 1932. His hammer is connected to the motor through helical springs that accumulate energy and deliver it as an impact on the tool head, and he states the object of the tool as a succession of impacts of great force. The court records that he and Holsclaw built 24 or 25 such wrenches without commercial success, and that in 1934 they interested Ingersoll-Rand, which licensed the patents. According to the court, Ingersoll-Rand then devised an automatic wrench that differed substantially in construction from the one in the Pott patent, largely aided by the Shaff and Fitch patents, and that tool did succeed, producing about $3,000,000 in royalties to the Pott and Holsclaw interests.

Credit for the first impact wrench is claimed in more than one place. Pott’s patents are the earliest documents in this entry. Ingersoll-Rand’s 1940 catalog presents the company as licensee of those patents and its 1959 catalog calls the company the originator of Impactools. Chicago Pneumatic’s own history says it designed and manufactured the world’s first impact wrench in the 1930s, in pneumatic and electric versions. These claims are listed under disputed. They are company accounts, and the court record is the only independent description of how the tool reached the market.

Other makers entered once the mechanism was commercial. Black & Decker, the court notes, studied the design of an automatic wrench for four years before marketing one in March 1949, and the court found its tool not to infringe. The 1951 decision also held the Pott patent invalid for want of invention in view of the prior art, a finding that rests on the court’s review of earlier impact tools and that this entry has not examined further. Ingersoll-Rand’s 1959 catalog shows that by then the same mechanism was offered in electric tools in sizes up to 1 inch.

The tool has since followed the power source. Corded electric models such as the DEWALT DW292 and DW297 give 325 and 434 foot-pounds in their manuals, and cordless models such as the Milwaukee M18 tools run the same hammer-and-anvil action from a battery. What the impact wrench displaced was the hand wrench and the sledge. Ingersoll-Rand’s 1940 catalog says it is no longer necessary to hammer nuts to loosen them.

Timeline

1932
Robert H. Pott files his first impact-tool application on 30 January (US 2,012,916) and a second on 6 October (US 2,049,273).1 2
1934
Pott and Holsclaw grant Ingersoll-Rand an exclusive license under the patent on 10 July, with a royalty of 5% of net sales, after their own tool failed commercially.1
1935
US 2,012,916 is granted to Pott on 27 August.1
1936
US 2,049,273 is granted to Pott on 28 July. Clifford E. Fitch files the application that becomes Ingersoll-Rand’s US 2,220,711 on 7 January.1 2
1930s
Chicago Pneumatic says, in the 1930s section of its company history, that it designed and manufactured the world’s first impact wrench in pneumatic and electric versions; the page gives no year.1
1939
Shaff’s US 2,143,173 for a rotary driving tool with impact action, assigned to William H. Keller, Inc., is granted 10 January.1
1940
Ingersoll-Rand’s Air Tools catalog Form 2341 A offers numbered impact wrenches, states the company is the only licensee of the Pott impact wrench patents 2,012,916 and 2,049,273, and says the wrench removes frozen nuts that a torque-type pneumatic wrench cannot start. Fitch’s US 2,220,711 is granted 5 November.1 2
1949
Black & Decker puts an automatic wrench on the market in March, after four years of design work.1
1951
The District of Maryland holds the Pott patent invalid for want of invention and not infringed by Black & Decker’s wrench, and dismisses Ingersoll-Rand’s complaint.1 2
1959
Ingersoll-Rand’s catalog of electric Impactools, headed “Originators of Impactools, air and electric,” lists electric models in several sizes up to a 1 inch drive.1

Where the sources disagree

Who made the first impact wrench, and when?

Robert H. Pott filed for the impact tool in January 1932; he and Holsclaw built 24 or 25 wrenches that did not succeed commercially, and Ingersoll-Rand licensed the patents in 1934 and developed the commercially successful tool.1 2
US Patent 2,012,916 and the 1951 court opinion in Ingersoll-Rand v. Black & Decker
Ingersoll-Rand describes itself as the originator of Impactools and, in 1940, as the only licensee of the Pott patents.1 2
Ingersoll-Rand trade catalogs
Chicago Pneumatic designed and manufactured the world’s first impact wrench, in pneumatic and electric versions, in the 1930s.1
Chicago Pneumatic (company history)

Was the Pott patent valid?

Ingersoll-Rand, Pott and Holsclaw sued Black & Decker for infringement of the patent and were paid royalties by Ingersoll-Rand of about $3,000,000 in total.1
Plaintiffs in Ingersoll-Rand v. Black & Decker, as summarized by the court
The patent is invalid for want of invention in view of the prior art, and if valid must be narrowly construed and is not infringed by the Black & Decker tool.1 2
Chesnut, District Judge, 1951

Gaps in the record (7)

The Fourth Circuit’s 1951 appellate opinion (192 F.2d 270) was not readable from here, so the outcome on appeal is not stated.
The prior-art impact tools the court relied on, and the identity of the earliest rotary impact mechanism, have not been traced.
Chicago Pneumatic’s 1939 claim has not been matched to a patent or catalog.
The date of Ingersoll-Rand’s first Impactool model is not established; the 1940 catalog is the earliest document read.
The Ingersoll-Rand catalog dates (1940 and 1959) come from the Internet Archive record, not from the printed catalogs, which show only form numbers.
Standard texts (IEC 62841-2-2, ASME B107.4, ISO 1174) have not been read.
No independent test of torque, durability or accuracy has been sourced, so buying is null.