Joshua Bigwood & Son Limited

Coal stokers

An underfeed stoker, located beneath the fire.

Underfeed stokers are a British invention which was used as early as 1838. It is an ideal way of firing bituminous coal in a small automatically controlled unit. The stokers have been used for such diverse purposes as the firing of central heating boilers, steam boilers, kilns and almost every type of industrial furnace. They provide an efficient way of firing which uses the minimum amount of fuel and requires less labour than traditional methods.

Bigwoods offered a range of underfeed stokers for various types of boilers. There was the 'Unicalor' stoker and the 'Unidrive' stoker.

In the 1980s both models were replaced with the Bigwood angle feed stoker. This was basically a bunker to boiler stoker but with the pick up screw capable of being angled through 180 degrees. In the 1960s Bigwoods were the largest underfeed stoker manufacturers in the country, with an annual production in excess of 600.

In addition, Bigwoods made screw elevators, turf burners for burning peat and trickle feeders for the top firing of brick kilns.

Unicalor coal stokers

The ‘Unicalor’ stoker was available as a bunker to boiler model. A screw inserted in the base of a coal bunker brought the coal forward to a transfer box, where a second screw collected the coal and delivered it to a retort set in the base of the boiler, where combustion took place. The stoker was available for coal feed rates of 220 to 750lb. per hour for firing tubular flue boilers. A similar design, suitable for installing in cast iron sectional and vertical steam boilers was available for the full range of 20 to 750lb. per hour.

The ‘Unicalor’ stoker was also available as a hopper model, which had a hopper for the coal situated over the transfer screw.


The photograph shows a Bigwood Unicalor screw type standard hopper stoker, applied to a small vertical boiler.

The Unicalor bunker-feed stoker

The Unicalor Drive

A 'V' belt drive transmits the drive from a totally enclosed electric motor to a worm reduction gear, from which the final reduction is obtained through an adjustable pawl and ratchet mechanism. This gives the choice of a range of feeds by adjustment of the selector plate or mask. All parts were interchangeable and a slipping clutch protected the mechanism from an overload. The gears were contained in an oil bath.
This is part of a Bigwood's advertisement that appeared in the programme of the St. Peter's Church Bazaar of 1938, which was held in the Civic Hall.

It shows Unicalor stokers in the then new Civic Hall and refers to another recent installation in St. Peter's itself.


Unidrive coal stokers

After World War 2 Bigwoods offered a lower priced ‘Unidrive’ range of bunker to boiler and hopper models.

A Bigwood Underfeed Unidrive Hopper Coal Stoker


a.  Hopper
b. Hopper base
c.  Cleanout duct
d.  Retort
e. Air duct
f.  Drive unit and control gear
g.  Spanner
h.  Coal tube (first section)
j.  Thermostat
m.  Clinker tongs and dust rake
n.  Cover
s.  Clinker tray

The coal is fed by means of a screw into the base of the retort, from where it rises into the combustion zone. The retort is surmounted by a row of tuyeres through which air is introduced. The fuel and air move together vertically upwards to produce efficient combustion. The fuel and air flow are automatically controlled to suit the requirements of each individual installation.
An underfeed stoker shown during fire cleaning.

A bank of Bigwood Unidrive Hopper Coal Stokers in operation.
The stoker was supplied with everything that was necessary for the installation, including fireclay, firebricks, sealing cement and electrical cable and fittings. The coal feed into the boiler is adjustable in multiples of 20lbs per hour and there is a fire kindling control that can be set to operate for several minutes every hour. The stoker also has a thermostat and a fan to control the fire.
A close-up view of the air duct and the driving wheel at the back of the stoker.
A Bigwood drawing of the arrangement for converting a Lancashire boiler into single flue, five pass working, which increased the boiler efficiency but reduced the output to 60% of the original rating.
We would like to thank David Evans for his help in producing this page.

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