The Knotts Company
Thomas Pumps

WOB-L Piston

Wob_L_Piston_1WOB-L PISTON

WOB-L piston pumps work to reduce a volume by a fixed crank-driven piston within a cylinder.

  • Compact, reciprocating design for vacuum or pressure
  • Oil-less, non-lube piston and cylinder
  • Lightweight die cast aluminum components or highly wear-resistant plastic
  • Single stage and multistage models
  • Balanced for low vibration operation
  • All wetted aluminum parts treated for corrosion protection from moisture
  • Long life, high performance piston seal
  • Permanently lubricated bearings
  • Flow rates up to 7.1 cfm (200 l/min), pressure to 160 psi (11 bar), vacuum down to 29.1 in.Hg (-990 mbar)


The WOB-L principle has a unitary piston rod that 'wobbles' inside the cylinder as the crankshaft rotates. Taking its name from its principle of operation, the WOB-L trademark was patented by engineers working at Thomas 40 years ago.  They provided a piston compressor radically different from any other design on the market at that time and remains a significant innovation in the field of compressor technology today.

Unlike the more conventional articulated piston pump, the WOB-L has no gudgeon pin (wrist pin) connecting the piston rod to the piston. Instead the piston and piston rod are a single item, usually a single casting. The piston rod is mounted to an eccentric bearing assembly, which in turn is mounted to the motor shaft to convert rotary energy from the motor into linear motion of the piston within the cylinder.

The piston is sealed to the cylinder by a flanged disk cup, that forms both a seal and mechanical guide for the piston. It runs without lubrication in contact with a low friction, surface coated cylinder of high heat conductivity. As the piston is driven up and down, air resistance on the upward stroke expands the disk's seal on the piston against the cylinder wall to increase its efficiency, while compensating for the 'wobble' action.

The oil-free operation makes the WOB-L piston extremely popular in the medical, automotive and beverage industries.  The pressure vs. flow and vacuum vs. flow characteristics are generally superior to those of diaphragm pumps. In use, WOB-L provides far greater flows at any given pressure or vacuum, to ensure a more consistent operation, to provide longer life operation by allowing a physically smaller pump to be used to perform any given task.

As a piston pump, an inherent characteristic of the design is the failure mode is a gradual fatigue, rather than a catastrophic failure mode which is a characteristic of many other designs.


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