by Daniel Pierre III, JN Machinery Corp.

So you want to buy a used conveyor oven.

f you haven’t thought of it yet, you may soon ask yourself, “Why not take advantage of the good deals for a used conveyor oven?” With the number of auctions, bankrupt­cies, and consolidations going on right now, there are a substantial number of 10-year plus ovens on the market. But, before you shell out your hard-earned money for “the deal of the century,” please remember that it’s a Iittle Iike buying a 10-year-old car with 150,000 miles: “You better know what you’re buying.”

Here are somethings you should look for and be aware of:

Drive Motors – Many older ovens have DC indirect­drive (chain-driven) motors. The sprockets and chainsare a nuisanceand the DC motors have brushes thatconstantly need replacing. In addition, these motorsusually have powdered metal and/ or acetyl gears thatare inefficient and rather noisy. Many of these motorshave also been discontinued, so the inevitable futurereplacement becomes a costly problem down the road.Current ovens use trouble-free, direct-drive AC motors with Hypoid gearing. In addition, most of thesemotors have built-in pulse generators that allow digital speed control function.

Speed Control – Older ovens have a DC control with a 270° potentiometer for belt speed adjustment (i.e. from slow-to-fast within one turn of the knob). This produces poor repeatability on setting cycle times and there is no compensation for voltage changes. Current ovens either have a lockable 10-turn potentiometer which gives wider speed control range with recordable settings, or a digital control that works with the motor pulse generator and automatically adjusts for motor RPM changes. Note: a digital speed control connected to the pulse generator is required to meet ISO and AMS 2750D requirements.

Oven Belt Height Adjustment – On older and smaller ovens, the belt height was adjusted through either a telestrut or a scissor design. After a few years these designs often became jammed and difficult to operate. Plus, they usually required assistance via a forklift and more than oneperson was needed to make adjustments. Current ovens have jacks mounted to each corner of the oven which allows quick and easy adjustment of the oven belt height. More importantly, one person can do it by hand.

Fan Motors – Smaller ovens (less than 12″ belt width) have fan motors that are directly connected to the oven fan. These motors on older ovens have a poor heat insulating rating and usually have a rather small (3/6″ ) diameter motor shaft. If the fan blade becomes bent or unbalanced, the small shaft is easily distorted and will cause vibration and failure of the fan motor. Also, just like drive motors, older fan motors may be unavailable for replacement. Today’s new, smaller ovens use a high temperature fan motor with at least a ½” diameter fan shaft for better stability and longer life.

Heat Shields – Most domestic conveyor ovens have a stainless heat shield that lines the heat chamber. The stainless shield not only provides a hard surface for circulating fan air to bounce off of, but also provides radiant heat off the stainless surface. If an older oven has been used to heat-treat excessively oily parts, the face of the heat shield probably has oil burnt onto the surface which eliminates the advantage of radiant heat. This dramatically reduces the efficiency of the oven.

Thermocouples – More than likely the thermocouple or thermocouples have never been changed. While the thermocouple may appear to be working, an old thermocouple becomes oxidized over time, which slows down the reaction time of the thermocouple to the chamber temperature change, causing greater temperature variation. It is recommended that thermocouples be replaced once a year to eliminate this variation. Ask your used oven dealer if the thermocouples have been replaced. That way you will know if this is an expense you will soon have to deal with or not.

Curtains – Old, frayed or non-existent curtains waste a lot of electricity and increase your operating costs by a larger degree than the cost to replace them with new curtains. This expense should be part of your calculation of the “real” cost of your used oven.

Elements – A 10-year-old oven, depending on its usage history, is probably going to need elements replaced in the near future. Element costs range from $30 to $100 per element, depending on the size of the element. Definitely plan on this potential, added cost in your “used oven” deal.

Oven Belts – Warped, stretched or torn belts might have to be replaced and are a very costly expenditure. Depending on belt size, costs can vary between $400 ~ $1,800 and may not be stocked by the OEM manufacturer (especially on older and discontinued models). In such cases, not only do you have an additional expenditure, you have a 2 or 3 week lead time to obtain another belt.

Manuals & Electrical Drawings – Have you ever had a manual or set of electrical drawings that haven’t disappeared after 30 days? It would be very surprising if one came with your used oven. Don’t expect the OEM to supply one at no charge.

Temperature/Cycle Time Charts – This is one of the most important things that you are not getting with a used oven. How do you know what temperature to set the oven at to achieve a part temperature for a specific cycle time? All new ovens come with this very important information.

Certification/Temperature Calibration – The used machinery dealer or auction house cannot supply this. Certification can only be supplied by the OEM or an ISO approved certifier.
Burnout Detection and Other New Features – New ovens take advantage of new technology as well as current requirements concerning safety and repeatability of the heat-treatment process. Some of these new features, such as Burnout Detection, cannot be retrofitted into older ovens without a major overhaul and/ or expensive investment in upgraded ancillary components.

Guarantee – Is the auction house or used machinery dealer giving you a guarantee? If so, what does it cover and for what length of time? Is your deal really worth the oven in “as is” condition?

After considering these points, do you still want to buy that used oven? Well, before you do, maybe you should call the OEM and give them the serial number of the oven you intend to buy. They should be able to tell you some history, concerns, and availability of replacement parts.

This information will help you find the real cost of the used oven. After all, would you buy a 10-year-old, 150,000 mile car without checking it out thoroughly? Why not do the same with a used oven?