What Size Convey or Oven do you Need?
By Daniel Pierre III, JN Machinery Corp.

It is a common misconception that conveyor ovens with similar belt widths are the same, with similar capacity; therefore the cheapest oven is the most economical choice for a particular application. However, there are many factors to consider when choosing a conveyor oven.

These springs will both yield 50 lbs./hr. of parts yet will require different sized ovens.

The obvious difference among ovens of similar belt width is the length of the heat chamber, which can vary dramatically. Thus, the area of the heat chamber often becomes the second consideration. Although this physical area is a much better criterion than belt width alone, there is still more to consider when sizing a conveyor oven.

Other factors (in no particular order) are: the overall footprint of the oven, the height of the entrance/exit, the power requirement to run the oven, and the orientation and location of the heating elements (or other heat source). There are additional considerations, such as maintenance, service and other non-qualitative factors, but leave these factors until the final analysis.

When a customer shows us a spring or its spec and asks for the best oven for that part, we would generally follow the guidelines shown in “How to size a conveyor oven,” page 43.

These springs are basically the same size, but will require different sized ovens.

There are good reasons to follow these steps in order. This process emphasizes the fact that the geometry of each spring plays such an important part in deciding what oven to use that it’s almost impossible to create a static table or computer program to choose an oven for you. Let me explain why each step is necessary:

1. Required information: We must look at both the physical size of the parts and their mass per unit time in order to fully understand how much work an oven has to do to get the job done. Very often, we get only the physical dimensions but no production-rate information. A tiny spring being spit out at 15,000 pieces/hr. will create a large mass of springs to heat treat each hour. Please do your ovenmaker a favor and include your targeted production rate when you inquire about a conveyor oven. An ovenmaker’s goal is to ensure the recommended oven will not interfere with the production process rate.

How to Size a Conveyor Oven 1. Required Information (Note: Without knowing a - f, we cannot size an oven ) a) Production Rate b) Wire Diameter c) O.D. d) Free Length e) Number of Coils f) Material (to determine heat-treat temperature and to weed out 17-7 steel; do not treat 17-7 on a conveyor oven) 2. Calculations a) Mean Diameter ( = O.D. – Wire Diameter ) b) Weight / Hour = Use Slide Rule or See Spring’s Spec sheet or = Mean Dia. x Number of Coils x CONSTANT 3. Assume Model Size (if you choose wrong size, you will notice at Step 5) a) Compare Wire Diameter vs. oven Capacity Chart (usually on a maker’s Web site) b) Compare Weight/Hour vs. oven Capacity Chart (usually on a maker’s Web site) 4. Calculate Available Cycle Time (use “inches”) [(belt width / OD) x (chamber length / FL)] = A = Number of springs that fit in the chamber in one layer. (Hourly production rate / A) = B = Number of turns the belt must make in an hour. (60min / B) = C = Number of minutes available to heat springs (i.e. “cycle time”) Check the validity of “C” above by estimating if the springs will naturally fill the belt. If the parts can lie in two layers without distortion or tangling, then multiply C by two. If you need 50% belt coverage to prevent tangling or distortion, divide C by two, etc. 5. Compare Cycle time with a Temperature Chart Ask...Can Heat Treat Temperature be achieved for final third of cycle time? (Will the curve be at or above the temperature for final third?) Ye...You have the correct sized oven. No...Can’t get to heat (less than ¹/3 of cycle time), there is too much weight going through the oven per hour. To use this oven, you will have to heat at a temperature much higher than the heat-treat temperature, which only decreases the useful life of oven parts – mainly the elements. Recalculate cycle time for the next larger oven size. You should find the curve fits for at least the final ¹/3 of cycle time. Yes, but...More than ¹/3 of cycle time. You over-estimated your oven size requirement. You will know this if you end up with a very long cycle time, 30 minutes or more, and find you get to heat for more than ½ of the cycle time. You can recalculate the cycle time for the next smaller oven size. However, there is nothing wrong with using an oversized oven, except that the operating cost will be higher than in a properly sized oven.

2. Calculations: Sometimes the technical information for a spring includes the developed length or perhaps even the weight of each spring. This is useful information, but most ovenmakers with springmaking experience will know how to make proper calculations.

3. Assume Model Size: Most ovenmakers will list the general capacities of each oven model by maximum wire size and/or maximum lbs./hr. in their product literature or Web site. Use these guidelines only as a starting point to size an oven. We have seen springs of such geometries that have changed the static-capacity rating by 400%. Usually, though, your initial assumed model size will be correct or within one model size of what you need.

These springs can be heat treated in two layers in this oven.

4. Calculate Available Cycle Time: This is perhaps the most important step, where one must use common sense as well as knowledge of the spring’s requirements and function. It does a springmaker no good if an oven can handle the calculated cycle time and heat-treat the springs if it yields distorted or tangled parts. “Cycle time” is the period of time from when the parts enter the heat chamber to when they exit the chamber. A good approach is to begin with the assumption that the springs will naturally spread over the entire belt surface when coming directly off a coiler. Then take a good look at the spring and how it is going to come off the previous process (coiling, shot peening, etc.) to determine how the parts will actually get onto the belt. Sturdy springs may be able to lay four or more layers deep, especially if the oven has heating elements on both sides of the belt. If the parts can easily tangle, you need enough empty space on the belt and consider low utilization, such as 40% belt coverage. Easy-to-tangle springs formed/coiled at high speeds can really increase your oven size requirements. If you have any uncertainty as to how close together your springs can lay on a belt, please send a handful of parts to your ovenmaker for specific testing.

These parts will tangle if they cover more than 40% of the belt.

5. Compare Cycle Times with Temperature Charts: Ovenmakers, through extensive R&D, will have several time/temperature charts that represent the rate of heat buildup in parts under loaded oven conditions. These charts are generated at various temperatures and at various belt speeds. A properly loaded oven will heat up parts quickly and then hold the parts at temperature for the remaining cycle time. The generally accepted theory is springs will be adequately stress-relieved if they are at temperature for the final one-third of the cycle time. The basis of this theory is once the very core of the wire material gets to temperature, more heating will not create additional stress relief. (At this point, I know many of you want to ask the “holy grail” question: How to convert batch time to inline conveyor cycle time. If you have read to this point in the article, I want to assume you agree with the general theory of inline stress relieving, so let’s reserve the batch vs. inline discussion for a future article.)

Comparing ovens and doing an analysis in this way will reveal a lot about an oven’s power and efficiency. If you do this analysis with different oven brands, don’t be surprised to find that an 8” oven from one maker will do the same job as a 10” oven from another maker. This revelation can save a spring company a lot of money if it can use a smaller oven, in terms of initial investment.

However, don’t stop your analysis there. There are final commonsense tests that bring all the non-quantitative factors back into play. Does it really make sense to save a few dollars on the purchase price if you will incur higher future costs, such as ongoing electrical usage costs, downtime waiting for spare parts or downtime due to extensive maintenance? These final factors can tip the scales in favor of one oven over another if you do find more than one oven that meets your requirements.

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Daniel Pierre III is president of JN Machinery Corp. in Bensenville, IL. Readers may contact him by e-mail at daniel@jnmachinery.com or phone at (630) 860-2646.