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Choice of guide: Round or square

Choice of guide: Round or square

2020-06-05

Since the introduction of the Ball Bushing linear bearing, a circular guide invented by Thomson in the 1940s, the field of circular Ball linear guide has been steadily developing.With the improvement of load capacity, service life and the simplification of manufacturing, the linear guide with near zero friction is used more and more widely.Since the introduction of square and linear guides in the 1970s, the key question for design engineers has been: should we use circular or square guides? 

Although various grinding requirements inherent in the design result in the initial cost of the square guide being several times that of the circular guide, new manufacturing techniques and scale economy have allowed the square guide to compete vigorously with the circular guide in a wider range of applications.After entering the field of machine tools, the use of square guides can now be seen in many applications where circular guides were once used only.However, sales of circular linear bearings are still increasing year by year.Due to certain inherent uniqueness and favorable characteristics suitable for specific applications, ball bushing guides are still popular, most importantly because they are round, and square pins sometimes do not fit into round holes. 

In order to apply the correct type of linear guide to specific applications, the design engineer must consider the advantages and capabilities of each guide.The first consideration must be the design requirements, which need to consider the load, accuracy, stiffness, finish, geometric size and other important factors.In general, there is an interaction between these requirements. 

Square guide products can well meet the needs of high load applications requiring high rigidity. For example, the maximum load of 25mm square guide can be up to 20 kN. When running in parallel between bearing guide and guide, it can reach the precision of 3 ~ 10m /m.The ball track in this guide product is precisely ground so that the guide is precisely matched with the ball, that is, the seat running on the ball guide can only be slightly larger than the diameter of the ball itself.This results in a greater contact area between the ball and seat under load than the circular guide assembly, where both the ball and the inner seat (shaft) are convex surfaces.Thus, as the ball flattens under load, the stiffness of the square guide bearing will increase by a factor of five when loaded. 

Square guide rails can also be used with pre-load conditions, ranging from light (3% of rated dynamic load) to heavy (13% of rated dynamic load).The typical application of square guide bearings is in the machine tool industry, which requires extremely high load capacity, stiffness and accuracy.Since the bearing has already undergone an initial deformation, the repreloading further reduces the amount of deformation under the load.So far, we have discussed bearings with spherical ball bearings.It is worth noting that square guide bearings with cylindrical roller type elements are also available if higher (more than twice) load capacity than square ball guide is required without increasing the envelope size. 

After the square guide can meet the load capacity, stiffness and accuracy requirements, many engineers will not conduct further tests.However, this results in reduced system performance and increased upfront and maintenance costs.The circular guide product has many advantages, one of which is that it can operate smoothly when mounted on an undersmooth surface with a flatness error in excess of 150 mm /m.When rigid linear bearings are mounted on rollers, at tilts, and at sideslip angles, flatness errors on the assembly surface often cause binding, resulting in higher towing and a reduction in service life of up to 50%.In order to solve this problem, in the assembly process, it is necessary to strictly trim the matching surface, or to adjust each part to the gasket and adjust it to the normal operation, which will increase the cost.For more sophisticated machining applications, up-front and operational costs are increased due to increased maintenance requirements.The cost varies widely from less than $100 to thousands of dollars.With circular guide rail products, the bearings can be installed to the welded tubular frame or directly fixed to the concrete factory floor. 

Linear bearings with ball bushings can be adapted to a variety of conditions due to their ability to adjust the center.Because the shaft is circular, the ball bushing rotates freely as it rolls around the shaft.The mounting height of the two adjacent guides may be slightly different, and a difference of 1 to 2 mm is acceptable when the guides are mounted at intervals of 300mm or more.Similarly, the housing rings on most ball bushing are designed to ensure that each linear bearing rotates up to 1/2 degree at the dip and slip angles to accommodate flatness or alignment errors.The shaft platform used to constrain ball bushing and guide rail support is usually made of aluminum, which has less flexibility to adapt to the maximum 25m parallelism error between each shaft.The result is an efficient boot with a friction coefficient of 0.001 to 0.004 under unsatisfactory conditions.The installation of linear guides to welded tubular frames is common in factory automation.In many OEM applications, the assembly surfaces are drawn tubes and sheet metal.Under these conditions, circular guides are usually installed more quickly and run more smoothly than square ones. 

A significant advantage of the circular guide product is that the circular plate can support the shaft at the end, realizing the circular linear bearing design that spans a gap of 12 to 24 times the diameter of the shaft. This design can be used in various applications from the gantry crane system in factory automation to the selection and placement of modules in DNA sample machines.The moving axis is constructed entirely by the two ends of the fixed axis.No matter what the machine surface between the two points is, no matter whether there is only one point, the precision of the device will not be affected, only depends on the accuracy of the end support device.In addition, if you want to use a linear guide to a set of screw drive system, you only need to punch three holes in a plate, that is, based on the support of two circular guides and the screw bearing, which is the easiest method to do.With this design, there is no need to adjust during assembly. 

The square guide solution to the alignment problem requires the use of only one guide.Since it can support torque in all directions, there is no need to use square guide bearings in guide pairs or even in bearing pairs.For light loads, i.e. loads within the bearing load and torque capacity, a single square guide bearing can provide the desired guidance, but not all square guide products can be designed for this type of operation.This guidance eliminates the problem of aligning the two guides on a gradually expanding surface.However, it must still be considered that the square guide is not designed to straddle the gap and end support by aligning through the transmission mechanism.Due to eccentric loading, a single guide may not be suitable for applications with a wide movement footprint, such as 300mm.Before making a decision, the torque rating of the guide and its stiffness under torque loads should be checked. 

Design flexibility is a very important consideration in component selection.Often, in order to achieve optimal performance, components must be modified to improve functionality or to match in the desired space.Square guide bearings are more compact than round ball bushing bearings when compared with load ratings.This way, if the geometry supports the user's system requirements, the user will have a fairly optimized shell.However, if modifications are required, this is easier to do on circular guide systems.The circular shafting can be configured in a number of ways so that the end of the shaft is fixed and can be used as a structural member in the large assembly, and can be centered and radial through the boreholes, planes and diameters of the axle box.With circular shafting, ball bushing can be purchased separately, which can be inherent in the boring of adjacent components, as well as aluminum or steel

The shaft table matches.The shaft table makes the bearing easy to install and allows a certain margin for modification. 

In some applications, flatness may be another major factor to consider.Ball bushing bearings tend to run very smoothly, and the difference between square and round guides is often felt by the hand, although there may be slight differences between the components and the application and there is usually a lack of industry-relevant flatness data.Point contact on the convex surface between the ball and guide minimizes the possibility of abrasion.Ball bushing usually operates under small clearance or light pre-load of 1 to 2% rated dynamic load, improving stability.Due to their high consistency and pre-load, square guide bearings can be used in some "notch" cases.In most cases, this is feasible, but if instrument grade bearings are needed, circular guides may be more advantageous.Also, due to its simple circular geometry, seals on circular bushing tend to be more reliable and produce less drag.The square guide wiper must conform to the ball track in the guide to achieve a durable seal. 

The last consideration when installing circular guides is the non-ball type polymer bushings on the shaft.High-performance bearing-class materials suitable for aluminum or other substrates may be used to provide good load capacity (equivalent to 20% of the load capacity of the ball bushing) and a lower coefficient of friction in the range of 0.05 to 0.25.Bushing bearings can operate smoothly in harsh environments with particles capable of damaging tightly sealed ball bushing, or can operate normally in ball bearing systems damaged by impact loads.Ball cycle noise, on the other hand, may be a problem in ball bushing or square guide bearings, but it is largely not in the case of these bushing bearings.There is no need for hardened steel shafts with ball bushing because the contact surfaces are distributed over a large area.Stainless steel shafting can be used for both clean environments such as semiconductors and pharmaceuticals and harsh environments such as food processing. 

In many cases, designers have great flexibility in choosing technologies.Fine-tuning a system's performance with the best components for each function produces the best solution, which translates into a competitive advantage in cost and performance.However, in order to achieve this optimal solution, total installation costs, as well as machining, surface finishing and installation costs, should be taken into account.


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