Linear Motion in Washdown: Select the right components for food-processing equipment
A washdown environment is one that utilizes, either by hand or by automatic means, cleaning with water, chemicals or a mixture of these. This washdown process can be as simple as a cloth and bucket or use of a hose to spray clean — or it can be under sophisticated high-pressure, controlled systems. The goal of these washdown operations is to eliminate bacteria and other microorganisms that can cause and spread disease. In recent years, examples of incidents such as e-coli breakouts and mad cow disease have rightfully led to greater scrutiny on processing equipment that may contain areas where unwanted bacteria can develop.
One key to successful linear motion design for a washdown environment is the choice of the materials used for the bearing, shaft or rail, and seal components. To achieve the requirements needed for corrosion resistance, proper standard and regulation compliance, and machine performance requires the right selection of materials.
Stainless steel is typically the preferred material for general use in direct food contact areas because of its corrosion resistance and durability. However, there are variations in stainless steel grades mostly in the levels of chromium and nickel.
• 300 Series: the most widely accepted material for food grade and medical applications. It is relatively soft, cannot be hardened and is also non-magnetic. Each of the grades (303, 304 and 316) can have different types that have slightly different formulations with varying strengths and weakness based on the addition to the mixtures.
• 400 Series: the most widely available and most used in industry is the 440, which can be heat treated and hardened. It is often used for cutlery, linear shafting and in applications requiring good wear resistance. It can be hardened up to approximately RC58; however, due to added carbon in its makeup, 440 will oxidize under washdown conditions.
Stainless steels do not rust with a red colored oxide on the surface the way that “rust” is normally observed. If these types of particles appear on a stainless surface, it is most likely due to iron particulate that has contaminated that surface or is coming from fillers within the bearing. To cleanse that surface, a solution of 10-per-cent nitric acid and two-per-cent hydrofluoric acid at room temperature can be effective.
Aluminum can be used in some areas of a washdown environment where weight is a concern. However, be aware that bare aluminum will have poor corrosion resistance and is susceptible to pitting and cracking. In washdown conditions, aluminum MUST be coated for protection. Often anodizing, ceramic coating, or other types of coatings with PTFE or other fillers are used, but may not provide the resistance or life that stainless steel offers. In more caustic chemical washdown environments, stainless steel is the preferred material.
Electroless nickel coatings have become increasingly popular because of their corrosion and wear resistance combined with a smooth polished appearance. Some forms include a PTFE infusion to aid in non- sticking properties. Most forms of this coating are FDA compliant as well.
Non-metal materials like plastics, polymers and fillers tend not to have the corrosion resistance and durability of metal surfaces over time and are thus not used as often as major components in food and pharmaceutical equipment. However, due to cost, weight, manufacturability, etc., they are increasingly being used “under the hood”, inside of mechanical drive components, guides, bearings, fasteners and more. Many solid plastics, such as injection molded bearing inserts, can present drawbacks in washdown applications in that most will absorb liquid, causing the component to swell and increasing the potential for binding and failure.
Also, be aware that each of the standards organizations covered earlier has extensive information on a variety of plastic materials that are acceptable.
Linear motion components offer their own unique challenges when being designed for washdown applications. Rotating components need to be mounted and sealed within a limited area, but because the moving component of a bearing, slide or actuator system travels in a linear fashion, the space needing to be sealed or cleaned will be far greater; often up to several feet. Below are some tips on how to minimize areas of potential bacteria buildup and maximize cleanability.
There are two basic types to be aware of when considering plain bearings. When using plastic inserts, be aware of moisture absorption that will lead to the bearing material swelling. This can result in binding issues. If the inside diameter is increased to deal with the swelling, it can often cause loose tolerances and inaccuracies in the system.
It is best to avoid open-ended bearings with grooves or inserts in areas that may be susceptible to bacteria buildup. These two-piece type bearings will allow the microscopic bacteria to seat in the crevices, grooves, and to hide between the outer shell of the bearing and the plastic bearing insert. One- piece bonded bearings eliminate this potential for bacteria collection.
The same principle is true for recirculating ball bearing type products, such as roundway linear ball bearings and profile rails. They provide advantages, such as low friction and tight tolerances, and are often available in stainless steel materials with FDA compliant lubrication. However, they can present disadvantages in that they require grease lubrication to be used due to the metal-to-metal contact. This lubrication picks up material from the food items being processed and can then become trapped inside of the multiple crevices and cavities around the balls and in the raceways of the bearing. This can potentially be a breeding ground for unwanted bacteria.
The best solution for most applications is to utilize a one-piece bonded bearing. The bearing materials, are PTFE based, self-lubricating and require no external lubrication that can collect potentially contaminated material. In addition, there are no grooves, crevices, or space between liner and bearing shell where residue can become lodged allowing bacteria to grow. The bearing material and outer shell are bonded together creating a true one-piece bearing.
In vertical applications such as those found on in-line and carousel bottle filling machines, it is advisable to utilize a bearing that is sealed at the top end. This eliminates contamination and the majority of fluid in the filling and washdown process from penetrating the bearing ID. Yet it allows the liquids that do get into the bearing system to easily flow through and exit at the bottom of the assembly.
Another area of potential concern in this type of configuration is that the many multiple component sub-assemblies utilizing a parallel shaft design can experience bearing binding problems due to misalignment. In addition, these multiple components are also susceptible to bacteria buildup around the connectors and joints. Newer technology that incorporates dual rail load capacities and functionality into a single rail design can eliminate potential areas of contamination collection.
It is best to avoid as much component assembly as possible in a washdown environment. Traditional methods for linear assemblies utilize a shaft and support rail bolted together, which are then bolted to a mounting plate or carriage. Each of these connection points creates a joint, crack, or crevice and a potential location where liquids can penetrate or where bacteria can begin to cling and buildup over time.
New technology in linear motion has created slide assemblies that eliminate the need for traditional multiple components and connectors. Unique two-piece slide systems are an ideal solution for washdown environments. In addition, these new style linear motion components are designed with smoothly curved edges that do not have recesses where buildup can occur.
When possible, avoid mounting connectors from the washdown side. They protrude and create another area where contamination can collect. It is best to bring the connector up through the bottom of the rail to be mounted. If necessary and connectors enter the washdown area, use a domed nut for easier cleaning.
Particularly in food grade applications, it is important to consider the location where the linear motion device is to be mounted in relation to the food being processed. When components that are not compliant or that do not meet other regulations for food contact, are used over the open food path or in a position where it could potentially come into contact with the food items being processed, risk can be eliminated by installing a stainless steel shield or cover over the components.
When constructing shields and other covers, it is important to give consideration as to how the panels and plates are to be connected or welded together. Small collection points for moisture and the potential for corrosion and bacteria buildup are the result of leaving the irregular surface of a weld exposed to the splash area. Whenever possible, the best-case scenario is to radius all corners.
Another tactic used to help in the management of moisture and fluids around linear motion components is to add weep holes, drainage channels, slots, or other porting features designed to channel the moisture away from potential pooling areas.
This is an edited article based on the PBC Linear whitepaper, “Linear Motion Design for Washdown Applications.” For more information, visit www.pbclinear.com.