Engineers tasked with designing heavy-duty lift systems are often looking for bearing systems that are capable of muscling heavy loads (over 1,000 lbs) while still offering a practical dose of precision. Often, to fill this requirement, engineers are quick to choose a classic product like profile rail. This choice is fine, but a closer inspection of alternative products like Hevi-Rail cam roller technology reveals that there are more practical and even superior solutions available in the engineer’s toolbox.
In this article, we briefly explore several heavy-duty product options for a real-world, robotic cantilevered lift system involving Z-axis bearings. We then focus on strategies for architecting higher precision into our mightiest cam roller, Hevi-Rail. Once readers understand these simple tips to optimal precision, we can then compare Hevi-Rail's might in motion with profile rail in terms of cost, materials, assembly, maintenance, and robustness.
We begin by using the real-world application example of a cantilevered Z-lift build-box [fig.1], which is essentially an automated forklift. In this scenario, higher precision is required, and defined by the deflection at the end of the fork resulting from the load, which has a max total of 1100 lbs, or 550 lbs per rail. The standard calculation for the Fmax in fig.1 confirms these loads. The deflection requirement is quantified as any play that is less than 0.5 mm. This is not an arbitrary number, as the function of the lift is to raise the build-box and then insert it into a very precise opening.
While all of the following potential solutions will excel at moving heavy loads, we prefer to think of each one as having a unique practical solution or niche. Keeping with the Z-lift theme, we have provided a quick run-down of each of the possible product solutions that were considered, as well as their corresponding application strengths:
Engineers may understand that Hevi-Rail does not have the inherent precision of profile rail or Redi-Rail, but they should be aware that there are straightforward steps to increasing fidelity to the level of profile rail. Once that precision is achieved, engineers can then have a powerful tool that offers heavy lifting with precision. Below are some basic principles that can be applied to this Hevi-Rail application to further increase precision and fall within the required deflection rate.
Use gravity to act as a preload, creating tighter precision. Installers can bias the assembly so that the clearance between roller OD and rail never comes into play [fig.2]. The bearing placement in the image has been exaggerated to show the bias. Actual clearances between the OD of the rail bearing (88.4 mm) and the inside dimension of the rail (89.4) for this Hevi-Rail product falls between 0.95 mm and 1.45 mm.
Use a third wheel to create a preload that will provide a bearing offset [fig.3]. This is a process used in the standard 3-wheel carriage for Redi Rail. Along with removing play, offsetting the bearing will also increase moment load capacities.
Machine the assembled unit versus machining each component and then assembling. Rails are hot rolled steel with a straightness of +/- 0.3 mm/mm and a twist of ½ degree/meter. For more information on preloads and machining, please read our blog Hevi-Rail Heavy Duty Roller Bearing Design Tips.
Structural Support Rails: The steel rails of Hevi-Rail can be used as the support structure, while profile rail must have a machined surface to butt against for alignment, and then finessed to reach ideal parallelism.
Ease of assembly: As previously stated, Hevi-Rail can be assembled as a support structure, and therefore set up as a single unit. A secondary advantage of the single unit is that once the structure is assembled, it can then be machined as a whole, facilitating high-precision alignment between the supports. In addition, Hevi-Rail uses a simple process of clamps or weldments between the joints.
With profile rail, alignment is much harder to achieve. In fact, many extra steps are required, including machined surfaces for rail mounting, high-cost drilled and tapped holes, and repeated fine-tuning and testing. These steps add up to a much higher tally in man-hours, and significant bottom line costs.
Environment: While Redi Rail and IVT have wipers and/or wheel covers to assist in dirty conditions, Hevi-Rail is capable of withstanding many contaminated environments such as garbage trucks, blueberry pickers, and outdoor scissor lifts. Their rollers have a sealed bearing design and are partially protected by the rails.
In contrast, profile rail has limitations in dirty environments. This is because the ball bearings are exposed to particulates that can cause premature wear and eventual catastrophic failure.
Cost: With the Hevi-Rail cam roller solution, there is a strong case to be made that its value in terms of high load tolerances and reduced cost and compares or exceeds that of profile rail. A quick comparison of components between the Hevi-Rail system and a competitor’s profile rail and block reveals a component savings of 20-percent. When you factor in that with the savings in assembly costs and the potential maintenance associated with dirtier environments, a much clearer strategy of implementing Hevi-Rail in precision-minded applications emerges.
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