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Plain bearings - Cost savings with cost efficient products
Industrial value chains are increasingly focused on manufacturing and maintenance costs. Maintenance costs represent a large portion of the ongoing operating costs of a system. High-quality and maintenance-friendly components help to keep these costs as low as possible while ensuring the required quality. However, planning and design does not always have to rely on the most expensive component. Using ABC analysis, products and components can be classified as ABC parts based on their value share and their importance in the value chain. The most cost-efficient articles are the so-called C articles or C-parts. These have a small share of value, but are required in large numbers. These parts have great potential to reduce costs. It’s worth taking a closer look. For many applications, cost-efficient items are a good alternative to expensive items. However, a low price does not have to mean a loss in quality; the application range of cost-efficient items may only be limited, for example, by the often lower precision. This article will show you how to use C-parts such as plain bearings in a cost-efficient manner, and how to select them purposefully, while at the same time ensuring quality.
What are C-parts?
What are C-parts really? By definition, C-parts are products that have a small share of value as part of material management and stock keeping, but play a large role in the value chain because they are required in large quantities. Due to their low value, a stock supply is usually created in order to reliably provide them in the production process. Good C-part management significantly affects the process costs of a value-added process. Thus, not only the optimal stocking quantity of these parts plays a role, but also their acquisition costs. Firstly, these can generally be reduced by increasing the procured quantity and the associated lower unit costs, but secondly also by purposefully adjusting the actually required product specifications of the procured parts to the intended use. An accurate analysis of the supply options, and applicable additional procurement costs and required properties of the part to be procured, such as precision and dimensional accuracy, is a prerequisite for uncovering potential cost reduction opportunities. Replacing a traditional item with a more cost-efficient item can be a possible step toward cost savings. MISUMI offers selected products designed to replace a traditional product with a more cost-efficient product without sacrificing proven quality. This is achieved by revising the tolerance range for use on medium accuracy requirements. Often, cost-efficient items also have a slightly reduced application range compared to traditional items. The cost-efficient items are identical in their basic form and structure to the existing items, but the tolerance and load limits differ from each other.
Quality assurance for plain bearings and C-parts
Is a cost-efficient plain bearing (C-value item) equivalent to lower quality? No, this is not the case. Cost-efficient plain bearings and traditional maintenance-free plain bearings from MISUMI both undergo a quality check before they are shipped. All shipped items meet catalog specifications.
- (1) Dimension test - inner diameter
- (2) Dimension test - outer diameter
- (3) Dimension test - overall length
- (4) Dimension test - external dimension
| Specifications | Internal diameter (mm) | Outer diameter (mm) | Total length (mm) | Permissible load |
|---|---|---|---|---|
| MPBZ12-20 | +0.034 +0.016 |
+0.018 +0.007 |
-0.1 -0.3 |
29.0 N/mm² |
| C-MPBZ12-20 (C-VALUE Item) |
+0.039 +0.011 |
+0.024 +0.003 |
+0.1 -0.5 |
20.3 N/mm² |
Cost Reduction Opportunities - Material Selection
There is also potential for cost savings when selecting the material of a plain bearing. Here, it is important to carefully consider in advance which technical requirements must be met and which materials and designs meet these requirements. The use of plastics, sintered metals, or simpler metal alloys are inexpensive alternatives to more expensive materials. When all required properties and requirements are met by the alternative material, simpler metal alloys or plastics in medium-duty applications can be a cost-efficient and effective alternative to expensive plain bearings.
The following describes a selection process for plain bearings:
Selection procedure for maintenance-free plain bearings
For maintenance-free plain bearings, the plain bearing bushings are soaked in lubricant. This treatment reduces relubrication effort. The applications for maintenance-free bearings are primarily applications with high loads and particularly demanding conditions.
Maintenance-free plain bearings and plain bearing bushings are selected in a three-step process.
In a first step, the operating conditions are determined: Is the plain bearing exposed to chemicals? How often is it used and what is the expected load? Does it have to be temperature-resistant?
The next step is to define the specifications: What is the best design for the application (e.g. with or without flange)? For example, a flange provides more stability and better load distribution, which is useful in applications with increased vibration levels. This step also temporarily selects the size and dimensions from the design conditions.
The last step is required to check the safety specifications: Does the plain bearing have a maximum surface pressure or speed rating that could potentially be exceeded in the application? What is the expected wear scope or service life? At this point, C-parts are usually different from regular products: These parameters are often minimally tighter on C-parts and are therefore not suited for all applications. However, in many applications, the given values will still be sufficient for use. If the review of the individual steps shows that the plain bearing is not suited, one must start over with step one.
The service life mentioned in the last step has a major impact on the selection of plain bearings. Moreover, service intervals can be better planned based on the service life and operational reliability can be increased. It can be calculated based on wear intensity W as a reference value.
The actual planned service life of the plain bearing and the resulting end of life depends on the application and accuracy requirement of the bearing. The load case also plays a key role for the type of wear and tear. If the shaft rotates in the bearing, an egg-shaped removal pattern occurs in the bearing. If the bearing rotates around the shaft, the removal will affect the full circumference of the bearing. Find more information about bearing load distribution in this blog.
The wear intensity W varies depending on the operating and environmental conditions such as operating behavior, surface pressure, type of movement, surface roughness of the contact shaft, and so on. If the wear intensity is proportional to the load and gliding distance, the following formula can be used to approximate the material wear.
W[mm]=K\times P\times V\times T
K is the specific wear rate. This can be found in the table below. T is the actual operating time or length of friction time. It is specified in hours.
P=\frac{F}{d \times L}
- P = surface pressure in N/mm2
- d = Inner diameter in mm
- L = Lnge in millimeters mm
- F = Force in N (Newton)
V=\frac{U \times n}{1000 mm/m}
- V = Cutting speed in m/s
- d = Inner diameter in mm
- n = rotational speed in revolutions/second [n/s]
Wear intensity calculation example
Assume a plain bearing with:
Inside diameter: d=20mm / Length: L = 10 mm / force (load): F = 1000N (without lubricant) / Speed: n= 2 revolutions per minute = 2 min-1 ≈ 0.033333 s-1 /Friction time: T = 100 hours
W[mm]=3x10^{-3}\frac{mm}{\frac{N}{mm^{2}}\times\frac{m}{s}\times {h}}\times \frac{1000N}{20mm \times 10mm}\times \frac{\pi\times 20mm \times 0.033333 s^{-1}}{1000\frac{mm}{m}}\times 100h
W[mm]=0.003\frac{mm}{\frac{N}{mm^{2}}\times\frac{m}{s}\times {h}}\times 5\frac{N}{mm^{2}} \times 0.002094 \frac{m}{s}\times 100h = 0.0031mm
The following table shows the range of the specific wear rate K of a plain bearing depending on lubrication:
| Lubrication | mm/(N/mm2 m/s h) | mm/(kgf/cm2 m/min h) |
|---|---|---|
| Unlubricated | 3x10-3 to 6x10-4 | 1x10-6 to 5x10-6 |
| Solid lubricant | 3x10-4 to 6x10-5 | 1x10-7 to 5x10-7 |
| Oil lubricated | 3x10-5 to 6x10-6 | 1x10-8 to 5x10-8 |
Application example for a maintenance-free plain bearing for medium-duty requirements
Below are two typical applications for maintenance-free plain bearings with different precision requirements:
- (1) Plain bearing variant C-MPBZ
- (2) Cylinder
- (3) Plain bearing variant MPBZ
- (4) Press fit
The application on the left has medium to low accuracy requirements. The better value (more cost-efficient) variant of the plain bearing can be used. The application on the right requires high precision linear/rotational movements that can only be operated with a regular (traditional) plain bearing.
Manufacturing costs saving on accessories and spare parts
Cost-efficient parts are not only available as plain bearings. The C-value product range also includes accessories and spare parts that are worth a look. Lubricants, screws, seals, and sealing rings may also be available as cost-efficient items and may be a good alternative for a specific use. Due to their standardization, they are available in large quantities and can be used universally. However, they are essential to the function and stability of a design.
Plain bearings are also often used in conjunction with pillar guides in guide systems for precise linear movements in machines and tools. Here, they reduce the friction of parts moving towards each other, thus enabling gentle movements. Low-cost plain bearings are suited for medium accuracy requirements.
How do I find C-parts in the store?
How do I identify C-parts in the product line? At MISUMI, C parts are sometimes identified by the part number, e.g. by a C prefix, as in C-MPFZ10-15 for a low-cost version of the plain bearing bushing MPFZ10-15. The C-part variant is approximately 50% cheaper than the standard version of the plain bearing bushing. The difference: For the cost-efficient C-part variant, the maximum permissible speed is 0.35 m/s and the maximum permissible surface pressure is 20.3 N/mm2, whereas the maximum permissible speed of the more expensive variant is 0.5 m/s and the maximum permissible surface pressure is 29 N/mm2. The recommended tolerance and operating temperature range are identical. Search suggestions for cost-efficient plain bearings and plain bearing bushings can be shown in the MISUMI shop by entering the letter combinations #CMP and #CMPF.
For a general search for cost-efficient parts, however, we recommend a search using the search function with the keyword "cost", as in this example search. This ensures that the items whose part number does not contain a C prefix are also displayed.
