These unsung heroes of the IV therapy universe deserve further study as they are not as simple as they appear. An injection molded cage with wheel guides molded into the sides to accept the axles of the adjustment wheel is the most obvious feature. When the wheel is positioned at the top of the housing an uncompromised flow of IV fluid ensues. The device must apply just the right amount of friction to the plastic tube to remain stationary while leaving the lumen of the IV tube wide open, not an easy task. The device then had to regulate the flow of IV fluid by changing the lumen of the IV tubing as the nurse turned the adjustment wheel.
There was strict dogma delineating the correct position of the clamp on the tubing. Our rigid, uncompromising diploma school instructors insisted on having the roller clamp positioned about 2 inches below the drip chamber so as to be able to locate it in the dark. Old nurses never, ever flashed on blinding overhead room lights at night. Sleep was revered as a healing agent. Having the clamp just below the drip chamber made it easy to locate at night.
There were 2 methods of establishing a flow control region within the clamp housing and one entailed the wheel traveling along an inclined ramp molded into the bottom wall of the device. The other entailed a clamp with the base parallel to the open wheel access port, not a ramp in sight! The wheel guides were inclined so the wheel traveled at an angle to the base of the clamp whereas the wheel was much closer to the bottom of the clamp when rolled down. The pressure on the IV tubing was varied by the up and down movement control wheel as it moved along the inclinations in the wheel guides.
Some really fancy roller clamps were dual action in that both a ramp and a variable inclinable control wheel changed the lumen of the IV tube. Some things in nursing made no sense and one of the most common places to find these over engineered, exquisite roller clamps was on clysis sets that were used to give fluids subcutaneously. Drip regulation on clysis sets was not a big deal, so why the fancy roller clamps? Some mysteries are never solved, especially in nursing, but that's a topic for another day.
Clysis sets were really old school and not subject to medical supply companies seeking to maximize profit margins by making cheaper roller clamps. Travenol clysis sets with dual action clamps even had metal axle spindles on the control wheels. I'm sure it would have been more cost effective to injection mold the wheel and axle in one piece. No cost was spared in producing these roller clamp gems.( For more foolishness on clysis: https://oldfoolrn.blogspot.com/2015/07/down-quart-hypodermoclysis-to-rescue.html . )
One of the problems with roller clamps was a phenomenon called control point drift which occurred when the control wheel spontaneously moved to a region of less pressure increasing the lumen of the IV tube resulting in increased flow rate. Definitely not a good thing, especially when titrating vasopressors. The dual action clamps with metal control wheels were rock solid once adjusted. The increased friction exerted by the metal spindles effectively attenuated control point drift.
Another problem with roller clamps involved the properties of the PVC intravenous tubing itself. After adjustment a phenomenon known as cold flow creep sometimes occurred. The lumen of the IV tube would decrease even though there was no movement of the control wheel. Lengthening the control zone on the clamp helped control this problem. Generally speaking, the longer the roller clamp the more stable the infusion rate. Short, stout roller clamps could be a real bear to regulate, some nurses even resorted to taping the control wheel in place.
The labor investment required by manual IV control was considerable because roller clamps needed frequent adjustment, but who would you like to see standing at your bedside, a concerned nurse or a noise barfing electronic poseur? Roller clamps also limited the number of IVs that could be managed on a ward. On a 30 bed unit the maximum number of running IVs was abour six. Roller clamps are long gone, but not forgotten.