Duplex Steam Pumps

6" or 8" Duplex pump

6″ or 8″ Duplex pump

The inspiration for this design came from a pump that was once produced by Robey’s of Lincoln. The tie bars overcame the problem of a complicated casting where both the steam and water cylinders are cast as one, a design usually used by other manufacturers of duplex pumps.

The valve gear, also Robey’s, is driven from the two piston rods and operates two slide valves, with the valve of one side controlling the cylinder on the other side, therefore a crossover system of passages is required between the cylinders and valve chest. Normally this means drilling holes at difficult angles and hoping they don’t break into each other. We have done away with all this by putting extra plates between cylinder block and valve chest. In these the passages are milled, enabling them to be in the correct position with the minimum of trouble. The pumps are double acting and self starting from any position. On the 8″ duplex the 8 water valves are the three wing type with the 6″ duplex using phosphor bronze or stainless steel balls.

4" Duplex Pump

4″ Duplex Pump

The drawings  have a parts list. Instructions are included on the drawings for machining steam and water cylinders to ensure that they line up with each other. As with all our equipment the pumps were first built up, run, stripped down, and the drawings produced from the parts.

For the 8″ Duplex we are supplying 16 gun-metal castings, drawn bar for the pistons, 10 silicone 0-rings for pistons & glands and drawings.

For the 6″ Duplex we are supplying 9 gun-metal castings, drawn bar for the pistons, 10 silicone 0-rings for pistons & glands and drawings.

For the 4″ Duplex we are supplying 7 gun-metal castings, 2 sizes of drawn GM bar for pistons and end covers, 12 O-rings and drawings. The design is slightly different from the other two in having different valve operating gear.

The Duplex pumps are supplied with phosphor bronze bar or gunmetal bar for the pistons and also cylinder end covers in the 4” and 6” version. We have been asked if we can supply the kits without these parts as often customers have this material or want to use stainless. Yes, we can supply kits without the bar giving you a saving in the cost of material and postage. See price list for details.


8″ Duplex 6″ Duplex 4″ Duplex
Approximate build up weight 6 lbs 2 1/4 lbs 1 1/2 lbs
Steam cylinders 15/16 in. dia. 5/8 in. dia. 1/2 in. dia
Water cylinders 5/8 in.dia. 3/8 in.dia. 5/16 in. dia
Stroke 1 1/2 in. 7/8 in. 7/8 in. dia
Overall length  (less fittings) 8 in. 6 1/4 in. 4 3/8 in.
Height   (less fittings) 3 1/4 in. 2 1/4 in. 2 1/8 in.
Width   (less fittings) 3 5/16 in. 2 5/16 in. 2 in.