A spare parts inventory is maintained to control or avoid the unacceptable consequences of equipment failure. In most instances the consequences will be loss of production or other loss of function which can be related to increased cost or loss of profit. In addition, spare parts may be held to manage consequences relating primarily to safety or environmental impacts.
The purpose of spare parts management is to optimise the competing aims of meeting demand while minimising the cost of the inventory. The main cost components associated with maintaining an inventory include:
downtime losses.
purchasing cost.
holding cost.
opportunity cost.
Many of the spares held will be for maintenance purposes and therefore these spares are usually linked to the assets to which they apply. A common method of linking the spares to the relevant asset is via. a 'Bill of Material' (BOM). These BOM’s comprise a listing of all parts of interest to the maintenance staff (not necessarily all parts in the machine) and those parts currently held in stock. While all parts held in stock should ideally be listed in the relevant BOM not all parts listed in the BOM are necessarily held in stock. The purpose of listing some parts without stocking them on site is to facilitate ordering of the parts when needed and to avoid delays due to confusion about the correct part.
Optimising the BOM’s and any stock holdings based on these BOM's is an important responsibility of the Maintenance or Reliability Manager as having spares on hand can significantly increases equipment availability while increasing the cost of spares held. Finding the optimum balance between the cost of holding spare part versus the cost of downtime or other losses resulting from not having the spare part available when required is the major focus of spare part management.
A major complication arises from the fact that the demand for spare parts is rarely constant or predictable therefore the demand is uncertain. The demand for spare parts can be considered as falling into one of two basic groups:
constant demand.
'ad-hock' demand.
Constant demand can arise from highly predictable failure rates (uncommon) and scheduled replacements based on routine maintenance or operational tasks which use specific parts or repair ‘kits’ during each activity.
'Ad-hock' demand arises from breakdown repairs or other un-predicted maintenance or operational activities. The increasing use of 'Condition Monitoring' in place of time based replacement has increased the uncertainty in demand for many parts. This is compensated for to some extent by the lead time provided by the 'Condition Monitoring' technique, in most cases allowing time to order the necessary spares. Where the procurement time is significantly less than the P-F interval obtaining the necessary spares should not present a problem for a well organized maintenance department.
In practice however a specific spare part can display a combination of both 'predictable' and 'un-predictable' demand as scheduled maintenance tasks interact with breakdown repairs both of which may require use of the same spare. Clearly the optimum stocking policy for a specific spare part will be highly dependant on the application and therefore each spare part must be managed individually to achieve the optimum strategy.
JAHCon has extensive experience in assessing spare parts needs and optimising spares holdings using established techniques as well as methodologies developed by JAHCon.
Go to OSL to see how using this tool can assist the maintenance team manage maintenance spare parts.
For further information on spare parts management and optimisation contact JAHCon using the details on the contact page of this web site.
