ABSTRACT
Executive Vice President,
Lubrication Engineers, Inc
Oven chains can be lubricated by a liquid or by a solid suspended in a fluid. Increased operational efficiencies can be achieved by selecting chain lubricants and methods of applying them that match the operating environments and failure modes of the chains. Several mini-case studies show that an application-specific chain lubricant designed to combat the deleterious effects of water, dust, heat, etc. can save a considerable amount of money.
Oven chains operating at high temperatures can be lubricated in two different ways: with a liquid lubricant, or with a solid lubricant suspended in a carrier fluid. Whether solid or liquid, the lubricating film physically separates contacting metal surfaces, thereby reducing friction and wear.
Fluid film lubrication
Fluid film lubrication is the regular oil film lubrication used in most ambient temperature applications. This type of lubrication can also be used at higher temperatures, provided that you take into account the decreased viscosity of the fluid (thinner oil film), the increased oxidation rate of the oil (can leave varnish and sludge), and the volatility of the oil (as it evaporates, there is less oil with which to lubricate).
A variety of synthetic base oils – polyalphaolefins (PAO), esters, silicones – allow lubrication at higher temperatures than can be achieved with petroleum oils. They have the same limitations (thinning out, oxidising and evaporating) as petroleum oils, but at higher temperatures.
At higher operating temperatures, frequent lubrication is required to replace fluid lost to evaporation. Evaporation, oxidation and decrease in viscosity make proper lubrication difficult to achieve with a liquid lubricant alone. In such cases, using a solid lubricant provides better wear protection and consumes less lubricant.
Solid film lubrication
Solid film lubrication is used at high temperatures or in applications where it is impossible or undesirable to use a liquid. The solid is usually suspended in a liquid carrier designed to disperse (evaporate) after the lubricant has been applied and, in the case of chains, after it has penetrated into the pin area of the chain. The example that most people are familiar with is graphite powder dissolved in kerosene, but technology has advanced well beyond this.
Solid lubricants provide a smooth, low drag surface, which reduces friction, wear, operating temperatures and electricity consumption.
The choice of the specific type of solid lubricant technology has an enormous impact on performance and should not be taken lightly. Materials commonly used as solid lubricants include molybdenum disulphide (MoS2, also known as moly), graphite (C), fluorocarbon polymers such as polytetrafluoroethylene (PTFE, commercially known as Teflon), metallic oxides, and a highly refined powder of aluminium, magnesium and silicate (Al-Mg-Si, commercially known as Almasol). Table 1 shows a comparison of these technologies.
On a microscopic level, all metal surfaces are uneven and have high and low spots. The high points, called asperities, on opposing working surfaces meet under heavily loaded conditions and the instantaneous contact temperatures of these asperities often exceed 500°C. Pressures in the contact zone can exceed 175,000psi (1). Only the Al-Mg-Si technology meets these needs and remains inert and intact under these extremes of temperature and load. Neither graphite nor PTFE has sufficient load-carrying capacities, and moly becomes abrasive, which causes wear. Consequently, NASA has used the Al-Mg-Si solid film technology on every manned US space flight including the lunar landings and the space shuttle. (6,7)
The size and the morphology of the particles of the Al-Mg-Si powder are carefully controlled. The particles are platelets, which form a single-layer coating on the surface of the metal through particulate attraction. The particles carry the load when the hydrodynamic oil film is gone (squeezed out or evaporated off), preventing metal-to-metal contact.
Reduction in friction
The reduction in friction translates into less wear, cooler operating temperatures and smaller electricity bills. The energy savings alone typically pay for the lubricant several times over. The most noticeable differences are the elimination of squealing and screaming of the chains. Another benefit is that the Al-Mg-Si powder does not build up on itself or make a mess like some other solid lubricants do.
Carriers
Different carriers allow the solid lubricant to be applied at different temperatures. For room-temperature application, light carriers such as kerosene are used. But if the chains are hot, the low flash point of these fluids is a fire hazard, so a variety of synthetic carriers are employed.
The best ones are smokeless and odourless. Unlike lubricants that rely on the fluid to provide the lubrication film, the evaporation rate of these products should be high rather than low because the oil is only a carrier to take the solids to where they are needed – inside the pin and bushing area.
The light carrier ensures proper penetration into the links of the chain at cool temperatures. During extended production runs, a smokeless, odourless, high flash point chain lubricant is applied to the hot chain as required. Because the chain is hot, this heavier carrier rapidly thins out and penetrates into the pin area of the chain.
When using lubricants containing suspended solids, it is important to agitate them before application. Many plants apply them to oven chains by hand using a garden hand sprayer (the type you pump, shake and spray) and apply them only once every week or two. There are also automatic lubrication systems, which circulate or agitate the fluid (to keep the solids in suspension) and then spray the fluid to the appropriate points on the chain.
When switching over from a liquid film lubricant, or when lubricating a new chain, the chain needs to be impregnated with the solid lubricant, initially requiring frequent application. Once the solids are in the chain, additional lubricant is required only to replenish the lost solids, and the amount of lubricant can be reduced significantly.
Frequency
The frequency and amount of lubricant required depends on the temperature and other aspects of the operation, but in general you should start lubricating every other day and extend lubrication intervals from there. Because most bakeries apply lubricant at weekends when the chains are cold, they typically apply a solid lubricant in a light carrier once a week, then extend this to once every two or three weeks. If you use a drip system you will have to apply the lubricant more frequently because it applies less lubricant at a time.
Another way of optimising the lubrication frequency and amount is to measure the current required to drive the chain. As the lubricant gets depleted, the friction, and hence the electrical power requirement, increases. This provides a useful feedback loop indicating that more lubricant is required. This is also a great way to evaluate chain lubricants, because the better they work, the lower the current drawn.
CASE STUDIES
Pretzel maker (Ohio)
The Rold Gold Foods division of Frito-Lay in Canton, Ohio, produces a variety of pretzels. One of its ovens has a chain drive kiln screen used in the baking process. The commercial-grade lubricant in use was black and messy. If it was not applied on a regular weekly basis, the chains would bind up, which in turn tripped the main power for the oven and shut down production. This problem occurred six to eight times a year. The temperature of the oven where the chains pass through is 246°C (475°F), with pretzel dust and salt present.
Since switching from the commercial lubricant to Al-Mg-Si, Rold Gold has never had a power failure due to build-up of solids or lack of lubrication. The company uses less lubricant than with the previously used chain lubricant, and they appreciate the improved cleanliness of the chains.
Bakery (Argentina)
Bimbo de Argentina, a large bakery in Argentina, operates Stewart ovens with skate wheel chains operating at above 200°C (392°F). The chains are lubricated with a centralised system. By changing the type of chain lubricant, build-up on the chains was eliminated, significant amperage drops were recorded and the amount of lubricant applied was reduced. The starting current dropped from 4A to 2A, and the operating current from between 0.70 and 0.90A to between 0.50 and 0.80A.
Bakery (Michigan)
Bluebird Baking Company is located in Detroit. It produces a variety of baked goods including hot dog and hamburger buns using Wilco tunnel ovens. The company was experiencing heavy carbon deposits on its chains and even on some of its baked goods. In 1989, the bakery decided to try an oven chain lubricant containing Al-Mg-Si technology. The change in lubricant resulted in a 26% reduction in power consumption (4.60 to 3.40A). This reduction amounts to more than $288 annually (£219) in energy saving on just one oven. The new lubricant also cleaned up the deposits from the chains.
Bakeries (New Zealand and the Netherlands)
Plants in both New Zealand and the Netherlands belonging to the Quality Baker group were experiencing expensive downtime due to chain lubrication problems. The messy solids containing the oil that they were previously using were carbonising, causing problems such as chain shudder, squeaking, and the chains jumping off sprockets. After converting to Al-Mg-Si oven chain lubricant, improvements experienced included the removal of carbon, no rusting or shudder, and in the Netherlands there was a 30% reduction in current draw (from 18.3 to 12.7A) resulting in excellent energy savings.
Conclusion
Significant operational efficiency improvements can be achieved by selecting chain lubricants and application methods that match the specific operating environment and failure mode of the chain. As illustrated by the case studies, an application-specific chain lubricant designed to combat the enemy (water, dust, heat, and so on) saves a considerable amount of money through reduced downtime, cleaning, lubrication, repairs, replacement chain and even electricity. Consider the factors that shorten your chain life and what you are going to do to prevent them.
References
- Errichello R., The lubrication of gears, Gear Technology Magazine 1991.
- Gaskell A., Molybdenum disulfide: New life for old technology, Lubricants World Magazine, August 1998.
- United States Steel,. Lubrication Engineers Manual, Appendix II.
- Calhoun S. F., Wear and corrosion tendencies of molybdenum disulfide containing greases, U.S. Army, August 1962.
- CRC Handbook of Lubrication, pp.269-276, Vol II, 1984.
- Neely R.J. and. Walden E O, ALMASOL – Key to new vistas of lubrication power, Engineering Magazine.
- Walden E.O., Various records, The Almasol Corporation.
Almasol is a registered trademark of Lubrication Engineers, Inc. Teflon is a registered trademark of E. I. du Pont de Nemours and Company.