Table 19.

Design limitations and implications.

Limitation	Description and implication
No fail-safe mechanism	If the escapement is decoupled from the leadscrew, there is currently no mechanism to prevent the delivery of all fluid in the reservoir. The escapement could become decoupled if the one-way bearing fails, the gearbox shaft slips in the gearbox connector, the gearbox fails, or the escape wheel slips on its shaft.
Excess spring force	As the compression spring is selected to provide sufficient force to back-drive the leadscrew when the plunger is at the minimum spring compression position, there is excess spring force at all other plunger positions. Extra spring force places greater load on key components such as the gearbox, increasing wear and reducing pump life.
Variable pumping pressure	Achievable pressure decreases from $\approx$175  kPa at maximum spring compression to $\approx$50  kPa at minimum spring compression. The large pressure drop is a limitation as if the required pressure exceeds the achievable pressure at any stage of delivery, the pump will fail to deliver. The pump will also not be suitable for a situation which must provide a constant pressure throughout pump operation.
Unsupported axial load	Only the axial load in the pumping direction is supported by the thrust bearing. The axial force on the leadscrew when loading the pump is currently supported by the gearbox connector and gearbox, which places unnecessary load on these components.
Limited pumping resolution	Although the pumping resolution of the design can be modified easily, it is unlikely that the resolution achieved by motor driven pumps with high gearing can be matched. For the majority of applications, the resolution achievable with the clockwork pump will be adequate.