This article provides a framework to evaluate when it is best to rebuild a rotary feeder, and when it’s time for a total replacement. It does a deep dive into how to evaluate your situation with the goal of helping you make a cost-effective decision.
Precision Machine and Manufacturing offers complete rebuilding and reconditioning of rotary feeders from any brand. We also offer new feeders for direct replacement.
Most Important Overall Consideration: The Condition of the Feeder
In wood chip or fuel handling systems, rotary feeders have been rebuilt for many years. If done well, rebuilding can restore a feeder to near-new condition for less than the cost of new. The condition of the feeder makes a difference in analyzing the rebuilding option. However, do not automatically assume that a well-worn feeder automatically tips the scale towards replacement.
All of the descriptions assume reputable, experienced, factory rebuilding of the rotary feeder. This includes new-vs-rebuild economics and other considerations. Rebuilds by a shade-tree operator not familiar with feeder engineering and design are an apples-and-oranges comparison to the services we offer. It will not achieve the results of a factory-rebuild.
Much of the cost and complexity of a rotary feeder rebuild arises from the condition of the feeder barrel (the concave portions of the feeder housing) and the work needed to repair it. Generally, there are two broad categories of rotary feeder rebuilds:
- Barrel-replacement rebuilds
- Bore-to-cleanup rebuilds
In either type of rebuild, the rest of the rebuilding steps are identical. The rotor is rebuilt. New knives, bearings, and brass seals are installed. The endbells are reconditioned, and the feeder internal clearances are set to factory standards.
In either case, the feeder should function exactly like a new feeder. Reputable manufacturers offer a warranty against any defects in material or workmanship that is identical to that of a new feeder.
A barrel-replacement rebuild is more extensive. In this case, the worn barrel is cut out of the housing using an oxy-acetylene torch. A new, pre-formed barrel is then welded into the housing. The material and labor required to cut out the worn barrel and weld in the new one increases the overall cost.
With a barrel-replacement rebuild, the feeder has a full-thickness barrel (typically 5/8” to 1-1/2” depending on the size of the feeder.) The thicker barrel provides a longer operating life before it is time for the next rebuild. Compare this to the bore-to-cleanup rebuild described below.
Costs vary widely. A rule of thumb is that a barrel-replacement rebuild is approximately 75-90% of the cost of a new feeder.
In a bore-to-cleanup rebuild, the feeder housing is chucked into a horizontal or vertical boring mill and a series of thin cutting passes are made around the internal diameter of the housing barrel. The machining continues until no gouges remain and the barrel is perfectly concentric.
The barrel will be somewhat thinner than full-thickness and, as a result, the interval until the next rebuild will be less than in a barrel-replacement rebuild. However, the cost will also be lower – approximately 60-75% of the cost of a new feeder.
This is the most common and simplest method to employ. It compares the ratio of the cost of the rebuild to the new feeder. Once the estimated cost of the rebuild is established, compare the cost to that of an identically equipped new feeder. For example:
|$21,000||cost of a rebuilding|
|÷||$31,000||cost of a new feeder|
Most often, a corporate standard exists for the threshold along the lines of “if the rebuild cost exceeds 80% of the cost of new equipment, buy the new item.” Thresholds vary widely. Some customers set the threshold as low as 50%. Other organizations set it very high in the belief that saving even a few thousand dollars is worth it.
There is a less-frequently-used variant of the threshold method that calculates the cost difference by subtraction and compares the cost saved against a minimum savings goal.
|$31,000||cost of a new feeder|
|–||$21,000||cost of rebuilding|
In this variant, the $10,000 savings calculated is measured against a corporate standard along the lines of “if the saving by rebuilding exceeds $5,000, proceed with the rebuild.”
Estimated useful life method
This is a somewhat more complicated method than the Threshold approach. It requires either good historical data or accurate estimating and judgment.
As mentioned above, in a bore-to-cleanup rebuild, the barrel will not be as thick as that in a new feeder or a barrel-replacement rebuild. For example, most 35×50 rotary feeders have a 1” thick barrel. If, after a bore-to-cleanup rebuild, a rebuilt 35×50 has a barrel thickness of 0.65”, it stands to reason that the feeder will not be able to operate as long.
Using this example, the rebuilt feeder would have a usable life of roughly 2/3 that of the new feeder or the rebuilt feeder with a new barrel.
Assume you have the operating records or the experience to know that a feeder with a 1” barrel will normally last 15 months in your plant before it requires rebuilding. Applying the 2/3 ratio to the bore-to-cleanup rebuild would indicate approximately 10 months of usable life.
For many plants a 10-month operating cycle will be just fine and the next replacement/rebuild can be planned accordingly. However, if the plant operates on a 12-month outage-to-outage program, the rebuilt feeder in this example could be problematic.
Discounted cash flow method
This method usually requires help from the accounting or finance department. It is most relevant to a longer-term planning and evaluation horizon. The two previous methods are well-suited to a here-and-now decision.
The discounted cash flow method evaluates the total cost of ownership of two or more operating and spending strategies over multi-year timelines. The strategies are then compared to one another in today’s dollars. The analysis requires a set of assumptions. The more accurate and refined the assumptions, the more powerful the method. A (very) simple example of this method is outlined below.
Assumptions in this example:
- A new feeder or barrel-replacement rebuild will operate 18-months in this plant’s environment before it needs replacement or rebuilding.
- The barrel in a bore-to-cleanup rebuild is half as thick as new and, therefore, will operate nine months.
- Only one bore-to-cleanup rebuild is possible before the barrel becomes too thin to be re-bored.
- A barrel-replacement rebuild costs 80% of a new feeder.
- A bore-to-cleanup rebuild costs 60% of a new feeder.
- The company’s cost of capital (discount rate) is 10%.
Strategy A – Always buy a new feeder; never rebuild
Strategy B – Rebuild every time it is possible
Each company could come to different conclusions based on the assumptions built into a discounted cash flow model. In this example, the arithmetic does not favor an “always rebuild” strategy as much as one might guess.
The discounted cash flow model can be significantly expanded beyond this simple model. Factors such as salvage value of discarded feeders, investment in spare feeders, or other considerations can be included in the calculation.
Other Decision Influences
While the primary framework for rebuild vs. new evaluation is most often financial, there are other influences that can factor into the decision-making process. Some common considerations:
A. Capital budgets vs. maintenance budgets
In many organizations maintenance budgets are controlled at the plant. Only local-level approvals are required to proceed with a rebuild. This is often the case even for a rebuild of significant expense.
B. Investment in spare feeders
Most feeder rebuilds, whether barrel-replacement or bore-to-cleanup, will require at least 5-6 weeks to complete. Therefore, the plant will need to have a second feeder available to install during the rebuilding period.
In theory, a plant that does not rebuild its feeders could forego the investment in a spare feeder by placing an order with the manufacturer for a new feeder in advance so that the new feeder arrives at the plant just in time to replace the worn feeder. In reality, most plants choose not to operate without a critical spare for fear of unplanned downtime.
C. Lead time
For a variety of reasons, plants may face a time challenge that tends to point one way or another in the new vs. rebuild analysis.
If the feeder is a size that is commonly held in ready-to-ship, new feeder inventory at the feeder manufacturer, the lead time can be as short as a few days. Rebuilds will normally take at least 5-6 weeks and can be much longer depending on feeder configuration and condition. Highly customized feeders, with upgraded barrels, for example, may require 10-12 weeks for the manufacturing of a new unit.
Rotary feeder rebuilding is a common, but often overlooked, alternative to buying a new feeder. There are numerous analytical tools and factors to consider in evaluating the choice. There is no standard, always-true conclusion about which is better, more cost-effective, or best-suited the company’s operation.
The Precision team is available to help as you are making your decision and planning the timing of your project. On request, we can provide budgetary quotes, estimated timelines for delivery, sizing and selection information, and more.
To get started, fill out the form below, call or email us at firstname.lastname@example.org. We welcome your inquiry.
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