Dempster Self-Oiling Windmills

Advertisement for the Dempster No. 12 Annu-Oiled Windmill, 1940s

NPS/Dempster Archives

The development of a windmill that could oil itself was the most important technological innovation in water-pumping windmills during the twentieth century. Self-oiling windmills had an enclosed gearbox with an oil-bath reservoir. As the windmill worked, the oil would continuously lubricate all the moving parts of the mill. This meant maintenance on these mills was minimal, only requiring oil about once a year. Until this time, windmill users had to climb windmill towers regularly to grease or oil all the mechanical parts.

The Elgin Wind Power and Pump Company of Elgin, Illinois, introduced the first commercially successful self-oiling wind pump in America in 1912. The main casting on this machine doubled as an oil reservoir. The moving parts of the mill brought oil to the other moving sections. This is the same principle used in the automobile engines. The Elgin customer only needed to climb the tower once a year to drain out the old oil and refill the reservoir.

Other windmill companies quickly realized that Elgin had developed something customers were willing to pay for. The Dempster Mill Manufacturing Company watched the developments taking place. They knew that their own older style wooden and open-geared steel windmills were becoming outdated. Customers did not want to have to climb their towers to lubricate their mills every week. The market died out for open steel mills, vaneless mills, and solid-wheel mills in the early 1920s, as consumers strongly preferred self-oiling mills.

Dempster Mill Manufacturing Company produced its first self-oiling windmill in 1921. The most popular and longest running windmill created by Dempster Mill Manufacturing Company was the No. 12 Annu-Oiled Windmill. It was manufactured for over eighty years with minimal modifications.

Dempster No. 11 Self-Oiling (1921-1929)

Dempster’s chief engineer, Henry Schlachter, experimented with designs for the creation of a direct-stroke self-lubricating windmill for years. He eventually recommended that the company use elements of an oil-bath windmill which the Aermotor Company owned the patent rights. Shortly after it was released, they were sued by Aermotor and bought a license for the manufacture of direct stroke mills under their patent.

The Dempster No. 11 Self-Oiling was a transitional windmill design that appeared in the 1920s and produced from 1921 to 1929. This was Dempster’s attempt to keep up with the trend of self-oiling windmills. It was designed so that it could use wooden or steel wheels and vane sheets.

The wooden No. 11 was made in ten, twelve, and fourteen-foot sizes. The steel pattern was produced in these sizes plus a smaller eight-foot diameter pattern. The wood and steel parts were interchangeable on the same sizes of mills.

The wheel on the wooden the No. 11 was similar to the earlier Dempster. No. 9 solid-wheel mill. The metal wheel was similar to the Dempster No. 10 mill, though the vane had sharp corners instead of rounded. The ungavlanized steel and cast-iron parts of the mill were painted black to protect them from the elements.

Lubrication was done by an “oil dipper” on the rocker arm. It lifted lubricant with each stroke from the oil reservoir into the main bearings. The oil would flow back to the reservoir to be used again in a constant cycle of lubrication. All the moving parts were protected by a galvanized sheet-steel hood, which has a distinct triangular profile.

The Dempster No. 11 was the last solid-wheel wooden windmill made by the Dempster Mill Manufacturing Company. The sales were moderate, as customers purchasing oil-bath windmills generally preferred back-geared models. The back-geared mills began operating in lower win speeds than the older-style direct-stroke mills like the No. 11. Therefore, Dempster began focusing on developing back-geared oil-bath designs, resulting in the Dempster No. 12

Dempster No. 12 Annu-Oiled (1922-2009)

Introduced in 1922 and in production until the 21st century, the Dempster No. 12 Annu-Oiled led the line of windmills produced by the Dempster Mill Manufacturing Company. The No. 12 was Dempster’s first self-oiling back-geared windmill.

The Dempster No. 12 first appeared in an eight or ten-foot-diameter Babbitt-bearing model in 1922. By the next year, an eight and ten-foot style equipped with Hyatt roller bearings replaced that pattern. A Dempster No. 12 mill with Timken roller bearings in six, eight, ten, and twelve-foot styles replaced this pattern in 1925.

The short-stroke twelve-foot mills were later replaced by a long-stroke twelve-foot mills in 1929. They also introduced 14, 16, and 18-foot patterns that year. An improved version of the eight-foot windmill called No. 12A mill was introduced in 1931. The windmills were further improved in 1982 with the No. 12B model. This version remained in production until the company stopped manufacturing windmills around 2009.

The main casting of the No. 12 served as an oil reservoir for the mill. The gears constantly dipped into this oil bath carrying the lubricant upward and splashing oil to lubricate all the moving parts. A galvanized sheet-steel hood protected the moving parts and the oil from contamination.

In February 1981, the manufacturer introduced a new diamond-shaped steel vane based on the design of the company logo. This design was not efficient. Vanes ideally should have a broad end for leverage to move around in the wind. The “Dempster Diamond” vane style was only produced for two years as it was discontinued in 1983.

Dempster No. 12 mills are some of the most frequently seen windmills. They can be seen in almost all parts of the United States and in many foreign countries.

Dempster No. 15 Annu-Oiled (1927-1941)

The Dempster No. 15 Annu-Oiled first appeared in a ten-foot size in 1927. This was followed by the introduction of eight and twelve-foot models the next year.

Many elements of the Dempster No. 15, such as the vane design and governing system were similar to the No. 12. The primary difference is the No. 15 was a direct-stroke windmill instead of a back-geared mill like the No. 12. Direct-stroke mills pump water every wheel revolution while back-geared mills need many revolutions per pumping cycle. There was debate between which was better as back-geared were thought to run better in lighter winds.

The blade design and angles were different from the No. 12. The wheel on a back-geared mill is designed to spin quickly, while a direct-stroke mill must turn slowly to develop the power and speed to pump water.

Lubrication on the No. 15 employs what Dempster called the “dip and step-up” system. The crank plate rotates in the oil reservoir. Special oil dippers on the plate lift the lubricant at the downstroke. The dippers bring the lubricant up to the other moving parts enclosed in the metal hood. After lubricating all these moving parts, the oil drains back to the reservoir.

Dempster No. 16 Annu-Oiled (1955-1960)

In 1952, Dempster decided to start production of a new windmill model. The Dempster No. 16 Annu-Oiled was released in a ten-foot size in 1955. This was the first new windmill model from Dempster in over 25 years.

The No. 16 Annu-Oiled was similar to the No. 12 with slight adjustments. The No. 16 reduced the number of wheel blades to help in high wind conditions. Another change was the use of bronze Oilite bearings instead of steel Timken bearings.

This model was meant to completely replace the No. 12. in the 6, 8, and 10-foot sizes. The 10-foot size was produced first because the 10-foot No. 12 pattern was worn out. The company planned to make 6 and 8-foot sizes after seeing how it worked in the field and modifying the design. Unfortunately, the new mill was not well received. The mill was never released in other sizes and the 10-foot No. 16 was taken out of production by 1960.

Sources

• Baker, T. Lindsay. 1984. A Field Guide to American Windmills. Norman, Oklahoma: University of Oklahoma Press. p. 192-197.

• Baker, T. Lindsay. 2010. "The Dempster No. 11 and the Introduction of Self-Oiling Technology to Windmills of the Dempster Mill Mfg. Co." Windmillers’ Gazette. XXIX, No. 4 (Autumn 2010) p.6-9.

• Baker, T. Lindsay. 2011. "When Marketing Overwhelmed Engineering: The Story of the 'Dempster Diamond' Vane Sheet." Windmillers’ Gazette. XXX, No. 1 (Winter 2011) p.8-9.

• Baker, T. Lindsay. 2012. "Solid-wheel Wooden Windmills Made by the Dempster Mill Manufacturing Company." Windmillers’ Gazette. XXXI, No. 4 (Autumn 2012) p.2-7.

• National Park Service. Homestead National Monument of America. Dempster Mill Manufacturing Company Records, 1878-2010.

Prepared by Amy Neumann, 2019