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Profile Rail Guides - Calculation of the Service Life of a Linear Guide
In industrial automation and mechanical engineering, profile rail guides are the backbone of precise and efficient motion. But how long do they really last – and what factors affect their lifespan? Read our blog to learn which types of service life are relevant, the advantages and disadvantages of profile rail guides, and how to maximize the service life of your equipment through precise calculations and targeted optimizations. Read on to learn how to make careful decisions to avoid operational downtime and ensure long-term productivity.
Advantages and disadvantages of profile rail guides
The high load capacity of the profile rail guides allows them to reliably support large loads, while their high rigidity ensures maximum stability and minimal deformation under load. These properties ensure excellent precision and accuracy, which are essential for high precision motion and precise positioning. Thanks to the low friction resistance achieved by the use of rolling elements, the guides move particularly efficiently. This both increases performance and reduces energy requirements. Complemented by their durability, achieved through high-quality materials and optimized construction, profile rail guides provide a consistently reliable solution that maintains their functionality and precision for long periods of time, even in intense operating conditions.
Despite their many advantages, profile rail guides also have some vulnerabilities that must be considered when designing the linear guide. As with any linear guide, the life of a profile rail guide is limited as the rolling elements and tracks are gradually affected by material fatigue and wear. They also show limited shock and vibration resistance, as high point loads or repeated shock loads can affect precision and smoothness. Excessive contamination from dust, particles or liquids can also interfere with low-friction motion, increase wear and even cause failures. To minimize these weaknesses and ensure long service life, careful planning, regular maintenance and the use of protective measures are essential.
In order to better assess the advantages and weaknesses of the individual types of profile rail guides, it is helpful to have a basic overview of the design and function of profile rails. In our blog on the basics of profile rail guides, you will find comprehensive information on their design, function and application.
Actual and nominal life
Two different values are distinguished when determining and evaluating the service life of profile rail guides, sometimes also called service life.
The "actual" lifespan (L)
Since the rolling elements and contact surfaces are subjected to a constant load due to the constant reciprocating motion under load, the linear guides suffer scaly damage due to material fatigue, which is called flaking. The total working distance until the first appearance of flaking is referred to as the service life (L), or also the service life, of the profile rail guide. The service life (L) thus describes the time or real travel over which a profile rail guide is used in a specific application before it must be replaced or overhauled. Life is determined by actual operating conditions and thus varies from application to application.
The nominal service life (L10)
The nominal service life (L10), partly also called nominal service life, is the calculated service life (achievable total working distance), which a profile rail guide will achieve with a 90% probability without showing signs of wear and tear and signs of material fatigue. The nominal service life in hours is shown with L10h.
Please note that MISUMI profile rail guides are calculated with a load rating based on 50 km displacement distance (C ≙ C50). DIN ISO 14728-1 and 14728-2 assume a displacement distance of 100 km when calculating. When calculating with 50 km, this results in higher load capacities compared to DIN ISO.
Calculating the nominal service life of a profile rail guide
In general, the calculation of the nominal service life (L10) of a profile rail guide is based on its load capacities and the actual loads caused by a payload. Depending on the application and approach, the nominal service life can be specified in load cycles, operating hours, revolutions or distance traveled. The type of bearing and thus the type of contact surfaces between the roller body and the running surface must also be taken into account when calculating the nominal service life.
Ball bearings and roller bearings have different contact geometries and load ratios. These differences affect the way loads are absorbed while also affecting the mechanical stresses that occur during operation.
This results in the following formulas for approximate calculation of the nominal service life (L10). Where MISUMI refers to a running distance of 50 km in the calculation and a constant load and travel speed is assumed.
for ball bearings
L_{10} = \left(\frac{C}{P}\right)^{3}\times 50km
- L10 = nominal life
- C = Dynamic load rating (N)
- P = payload (N)
for roller bearings
L_{10} = \left(\frac{C}{P}\right)^{\frac {10}{3}}\times 50km
For both ball bearings and profile rail guides, the load is transferred to the tracks via rolling elements (e.g. balls or rollers). Life is significantly affected by the contact stresses at these points. For more information on ball bearing load sharing, check out our blog about the load distribution of ball bearings.
Dynamic load rating C
The dynamic load rating C specified in MISUMI profile rail guides corresponds to the constant force, in direction and magnitude, that can be applied when testing a sufficiently large quantity of profile rail guides of the same series with a travel of 50 km each under the same conditions, without causing damage due to fatigue from rolling contacts in more than 10% of the profile rail guides tested.
The dynamic load rating refers to a defined distance of either 50 km or 100 km. The load rating C specified on MISUMI profile rail guides corresponds to the load rating C50. A dynamic load rating based on 100 km is given as C100 .
The load rating is one of the basic pieces of information for preselection and is provided by the manufacturer of the respective profile rail guide. You can also find more information about dynamic and static load capacity on our blog about permissible loads of linear guides.
Payload and dynamic equivalent load
For the basic calculation of the service life (L10), the payload (P) acting on the profile rail guide is also required. The formula for the approximate calculation assumes a constant load and speed. Profile rail guides are also loaded dynamically. Therefore, the dynamic influence and its impact on forces and moments are also considered.
Dynamic influences such as stepwise fluctuating loads, inertia, etc. can either be simplified via factors (coefficients) or taken into account by a more extensive load calculation. The payload (P) and dynamic equivalent load (Pm) calculations can be found on our blog about permissible linear guide loads.
Lifetime calculation taking into account the influencing factors
In order to take into account the real influences on the nominal service life (L10) as much as possible, influencing factors are used. These correction factors (coefficients) include the hardness coefficient (fH), the temperature coefficient (fT), the contact coefficient (fC), and the load coefficient (fW).
A lifetime calculation can help you plan your maintenance intervals and help prevent unforeseen failures and bearing damage. For more information on how bearing damage occurs and what symptoms and damage patterns they have, check out our blog about bearing damage.
for ball bearings
L_{10} = \left(\frac{f_H \times f_T \times f_C}{f_W} \times \frac{C}{P}\right)^{3} \times 50km
for roller bearings
L_{10} = \left(\frac{f_H \times f_T \times f_C}{f_W} \times \frac{C}{P}\right)^{10/3} \times 50km
Factors for calculating the life of profile rail guides
The following various influencing factors can be used to simplify the calculation of the nominal service life of a profile rail guide. They play an important role in adapting the approximately and theoretically calculated service life to the actual operating conditions.
Hardness Coefficient
The hardness factor or Hardness coefficient f H takes into account the surface hardness of the tracks and rolling elements.
Higher hardness means that the tracks are less susceptible to plastic deformation or premature wear. This allows for longer service life.
Temperature coefficient
The temperature factor or Temperature coefficient f T takes into account the influences of elevated temperatures on the material properties. At normal operating temperatures below 100°C, f T = 1.0 as material hardness and lubricant properties remain unchanged. Temperatures above 100°C can reduce material hardness and shorten guide life.
Contact coefficient
The contact factor or The contact coefficient fC takes into account the effects of the interaction of several guide carriages. In practice, 2 or more guide carriages are often used on the profile rail guide. However, since the load is not always distributed evenly across all guide carriages during machining, the load allowed per guide carriage also varies. Poor load distribution results in higher local loads and significantly reduces service life. For a load distribution on a single guide carriage, the contact coefficient fC = 1.0.
| Number of linear guides on the shaft | Contact factor fC |
|---|---|
| 1 | 1.00 |
| 2 | 0.81 |
| 3 | 0.72 |
| 4 | 0.66 |
| 5 | 0.61 |
Load Coefficient
The load factor or The load coefficient fW takes into account fluctuations and additional loads that can result from dynamic effects, shock loads, or unforeseen forces. Under ideal conditions and with purely static loads without fluctuations, fW = 1.0. However, in real-world applications, peak loads, vibrations, or irregular loads often occur, which can lead to higher values.
| Operating conditions | Load factor fW |
|---|---|
| No shocks/vibrations low speed: 15 m / min or less |
1.0 to 1.5 |
| No significant shocks/vibrations medium speed: 60 m / min or less |
1.5 to 2.0 |
| With shocks/vibrations high speed: 60 m / min or more |
2.0 to 3.5 |
Service life calculation in hours
Using the distance traveled in a given time, the service life can be calculated in hours. The calculation of the nominal useful life of a profile rail guide in operating hours is a common alternative to specifying travel, especially if the actual operating time and speed are of interest. This method allows for a more accurate life estimate in real-world applications because it is based on operating conditions. The nominal service life in hours can be determined using the formula below, which assumes that stroke length and stroke cycles are constant.
L_{10h} = \frac{L_{10}\times 10^{3}}{2 \times ℓ_s \times n_1 \times 60}
L10h = nominal life (hrs.)
L10 = nominal life (km)
ℓs = stroke length (m)
n1 = reciprocations per minute (cpm)
Precision systems such as linear ball bearings also use similar formulas to calculate service life. To learn more about the application and properties of linear ball bearings, read our blog about the use of linear ball bearings and linear bearings in guide systems with the highest precision.
Alternative Life Calculations
As explained earlier, the life of profile rail guides can be specified in different units such as load cycles, operating hours, distance traveled or revolutions. Each viewpoint has its own scope, which is determined by the nature of the movement and the requirements of the application.
The life in load cycles indicates how often a profile rail guide under a defined load can perform a complete load change movement (e.g. a back and forth stroke) before wear limits are reached. This specification is particularly precise, as it relates directly to the technical load capacity and allows for comparability with other profile rail guides. However, for applications with variable stroke lengths or irregular movements, conversion to other units is required to illustrate life.
The service life in revolutions is particularly relevant for rotating systems. The calculation is relatively simple when rotational speed and load are constant. For purely linear applications or complex motion combinations, this specification is not suitable and requires conversions.
