Pleasanton, CA-For years now, Mallinckrodt Inc. has been selling its well-known Puritan Bennett respirators, used in hospital emergency rooms, to help people breathe and to give them purer oxygen.
But when it started looking for ways to sell the respirators at a more affordable price, it ran into a classic engineering dilemma: how to lower the cost without sacrificing the accuracy. Healthy people breathe at a pressure of just under 0.5 psi, and hospitals find it essential to control this number carefully in life-and-death situations. When patients are badly injured or deeply anesthetized, and require a respirator to breathe, a leaky pump with falling pressure could be a serious problem.
"To be sold into markets that could not previously afford a product of this sophistication, the pump needed to achieve a high degree of accuracy and response, two of the critical functions in ventilation," says John Power, former R&D director at Puritan Bennett's facility in Galway, Ireland.
| Puritan-Bennett 740 ventilator system made by Mallin-
ckrodt, uses ball bearings to create a near-frictionless pump with a slow, † purposeful leak.
The standard way to make a precision pump--a tightly-sealed piston and cylinder assembly--is expensive to make and costly to repair when the seal starts leaking from the friction of constant use. So Mallinckrodt designers looked at an unconventional design; instead of using a ring seal around the shaft that drives the pump's main piston, they began experimenting with linear bearings from Thomson Industries Inc. (Port Washington, NY). The bearings had great endurance--wear and friction problems virtually disappeared--but they leaked far more than the seals had.
"The engineers were at their wits' end how to come up with a bearing to suit their needs," says Alison Ng, chief engineer for Round RailTM bearings at Thomson. "Standard bearings are not accurate enough."
So Ng adopted Thomson's high-precision, segmented bearing technology for a patented new use--a low-friction pump that is designed to leak intentionally, then increases air pressure with a faster piston speed to make up the difference--the Puritan-Bennett 740 ventilator system.
Thomson's segmented bearing technology allows the company to break the bearing down into its basic components, and change only the necessary parts. "The Puritan Bennett engineers said, 'OK, let's live with the leakage, but we'll increase the pressure,'" Ng notes. "And the only way to control that leakage is to know the 'annular area' between the outside diameter of the piston and the inside diameter of the cylinder." Since the piston shaft is so tightly controlled by the bearings, the piston never contacts the cylinder wall. So it causes no friction, and this area remains constant over time, allowing designers to measure the precise rate of leaked air with a closed loop feedback system.
"We're not doing a pure sealing, we're just discouraging leakage," Ng says.
The pump's 55-micron annulus height--about the thickness of a piece of paper--calls for very careful construction of the 167 mm (6.57 inch) diameter cylinder, explains John O'Mahony, controls and pneumatic manager at Mallinckrodt's Carlsbad, CA, location. That combination of close fit and low friction allows the pump to last for a lifetime of 60 million cycles at a mean pressure of 0.57 psi.
The pump has a 2.6-liter piston stroke displacement, which works well physiologically since the average adult lung capacity is 0.5 to 1 liter, says O'Mahony. The piston's shaft slides on Smart MultiTrac Ball Bushingģ bearings, which permit a purposeful leak of 8 liters/min. at the mean pressure level.
"We said, 'Guess what? We know things leak. So let's make the problem part of our solution,'" says Ng, of his work with the Puritan Bennett engineers. "To a mechanical engineer, a non-wear pump is a fairly radical application."