Doorknob

views updated May 23 2018

Doorknob

Background

There are 114 million existing doorways in the United States, with about two million new ones added every year. Doors equipped with suitable hardware are used to close off these openings and protect the interior of the building from the environment. Very early doors were merely hides or textiles. Wooden doors were also popular in ancient Egypt and Mesopotamia. Other materials used for doors include stone, metal, glass, and paper. Doors open by swinging, folding, sliding or rolling. Many swinging doors are installed with a lever or doorknob to open them with.

Door knobs have been used around the world for centuries, and were first manufactured in the United States in the mid-nineteenth century. Though spherical or ball-shaped door knobs are considered the hardest to turn, this shape is still the most common. Egg-shaped door knobs are the easiest for most people to use. Doorknobs have been made of many materials, including wood, ceramic, glass, plastic and different types of metal. Brass is one of the most popular materials because of its excellent resistance to rust.

The average doorknob is 2.25 in (5.715 cm) in diameter. The basic components are the knob rose, shank, spindle, and knob-top. The knob-top is the upper and larger part that is grasped by the hand. The shank is the projecting stem of a knob and contains a hole or socket to receive the spindle. The knob rose is a round plate or washer that forms a knob socket and is adapted for attachment to the surface of a door. The knob is attached to the spindle, a metal shaft that turns the latch of the lock.

History

American doorknob designs and materials have changed throughout the years. In colonial times the first door hardware was made out of wood, and involved simple latches and strings. Round knobs first appeared around the time of independence. Decorative hardware, including knobs, emerged after the Centennial Exposition of 1876. Before this time, most door hardware was imported; 95% in 1838.

Glass knobs were rare until a faster and cheaper manufacturing method was developed based on pressing. Pressed glass knobs were popular from 1826-1850, followed by cut glass through 1910. Wooden knobs were introduced in the late 1800s and were phased out after 1910. China or ceramic knobs were mainly imported from France and England until the mid-1800s, when the first U.S. patent was granted for making knobs out of potters clay.

Before 1846, metal knobs were made from two pieces brazed together or three pieces soldered together. Cast metal knobs were introduced around 1846. In the late 1800s, composite metal knobs were introduced as a less expensive knob. The main body was made out of iron or steel, covered entirely or in part with a veneer of bronze or brass. During the last half of the nineteenth century, many patents were issued regarding the spindle methods of attaching metal knobs for lock use, as well as designs for ornamenting these knobs. In 1870, a compression casting method was introduced that accelerated ornamentation of hardware.

Many of the Victorian doorknobs were made of cast bronze with ornamental patterns. During this period, a dozen major companies and many smaller firms produced hundreds of patterns of ornamented hardware, in addition to cast and wrought metal, glass, wood and pottery knobs. From 1830-1873, there were over 100 U.S. patents granted for knobs. Collectors have catalogued over 1,000 antique doorknob designs into 15 types based on shape, material, and design pattern. The best grade of knobs during this period were usually made from cast bronze or brass.

Around 1900, cast metal and glass knobs were introduced that incorporated ball bearings in the shanks of doorknobs. In operation, the knob shank rotated on sets of ball bearings fitted in the hardened steel cones. This reduced friction, assured closer adjustment, and eliminated endplay of the knobs. Other materials popular during the early 1900s included bronze and porcelain.

Most doorknobs come with some type of locking device. Machine processes for steel locks were first introduced in 1896. Today, the most common type of privacy lock is the spring lock, which uses a simple round, push button located in the center of the knob to control the bolt. It is easy to operate with a finger, closed fist, or elbow. Some locks come with both a spring lock and a dead bolt, which is operated by a key. Other locks have become more sophisticated, and use some sort of electronic device, such as a programmable computer chip that identifies users.

Design

Door hardware selection is usually based on appearance, cost, and availability, rarely on function. In order to assure the most usable hardware, designers must carefully consider not only appearance but also the size, shape, and feel of each element of door hardware and how easy it is to use. Typical design features of a doorknob include: no sharp edges or ridges; a shape that is easily grasped or turned; a textured finish or non-slip coating on knob to improve grip; and a shaft long enough to fit hand behind knob.

If a new design is required, a two-dimensional model is usually made using computer-aided design software. A three dimensional prototype is then fabricated so that a mold or die can be made of the desired shape. If a metal casting process is used, a pattern in wood or clay is made from which to make the mold.

Raw Materials

Most doorknobs are made of metal, with the most common type brass. The term brass refers to a group of alloys that contain a combination of varying amounts of copper and zinc. The material is usually received as a rod or billet of suitable diameter and is machine cut to the required length. The raw material must conform to standards developed by the American Society of Testing and Materials regarding physical, mechanical, chemical, thermal, and microstructural properties for each specific process.

The Manufacturing
Process

Though there are several processes used for metal doorknobs, including casting where a molten metal is poured into a mold, brass doorknobs are typically forged. Forging is a process in which heated metal is forced into shaped dies under very high pressure. Forging can produce products having superior strength, toughness, reliability, and quality (up to 250% stronger than castings). Forging can also be more efficient and economical.

Forging

1 The billet must first be heated to 1,400° F (759.9° C) in a gas-fired furnace to soften the metal. The billet is then placed in a specially designed set of steel dies that are shaped to conform to the profile of the product being made. A press applies high pressure to force the heated billet into the die cavity, where the billet now takes the shape of the doorknob.
2 After the doorknob is ejected from the press, it is cooled, trimmed to size, tempered, and cleaned to remove heat scale produced during the process.

Surface preparation

3 Next, the doorknob goes through a series of finishing steps. Separate coining, milling, drilling, and tapping processes produce a surface ready for the polishing operation. One or more buffing steps are used to achieve a brilliant finish.

Coating

4 For further protection, an organic or inorganic coating is applied using several different processes. Organic coatings include polyurethanes, acrylics, and epoxies. Because the solvents used in organic coatings can produce hazardous materials and quality problems, manufacturers are turning to inorganic coatings based on inert metals. These are applied using electroplating or physical vapor deposition (PVD).
5 PVD applies a coating produced by sputtering and thermal evaporators in an airtight chamber. The chamber is evacuated to high vacuum pressures (less than one millionth of an atmospheric pressure) by a series of pumps. A thin coating is deposited one molecule at a time. For successful PVD, the brass surface must first be extensively cleaned in a series of washing and agitating tanks, followed by electroplating with semiprecious materials.

Quality Control

The raw material must be of suitable composition for the forging process, as established by the American Society for Testing and Materials. Various process parameters throughout the manufacturing process are monitored and controlled to ensure the final product meets quality standards. The finished doorknob is inspected for dimensions, surface finish, and other properties. Some of these properties may have to conform to certain building codes.

Byproducts/Waste

Since forgings are designed to approximate final part shape, little waste is produced compared to other processes. The forging process also results in uniformity in composition, dimensions, and structure from piece to piece and lot to lot, which also minimizes rejects.

The Future

Security and access control systems for doors will continue to become more sophisticated as the cost of electronics decreases. Though there will always be a demand for mechanical hardware, electrical hardware may have faster growth. New building codes may be required to accommodate this hardware.

The common doorknob will continue to play an important role in the building industry. Restoration and renovation of older buildings will continue to make antique doorknobs or their reproductions popular.