A bas-relief dated to 650 BC Iraq depicts heavy loads being transported with rollers, proving that the technique of rolling rather than sliding is at least 2,500 years old. Also of interest is a thrust bearing found aboard a Roman emperor's ship salvaged from a lake near Rome. This object, thought to have been made c. 50 AD, is strikingly similar to the rolling bearing of today. Judging from these artifacts, we can see that the principle of the rolling bearing was practical "technology" in the distant past.
When we consider that heavy goods have long been transported over long over-land distances by man power alone - before there were dump trucks and cranes - we know that rolling must hold some secrets.
Let us consider this a little more. When a 1000-kg object is placed on the ground, how many kilograms of force are required to move it? We can come up with a number of answers, such as 50 kg, 100 kg, 300 kg, and 1500 kg.
Why are there so many answers?
There are so many answers because the required force depends upon the features of the surface in contact with the load. The force is 300 kg if the surface is conventional pavement, 50 to 100 kg if it is sandy, and 1500 kg if the surface is soft. When a force is applied to an object at rest, as in this example, the critical applied force at the point at which the load is just about to begin to slide is called the maximum static frictional force. In many cases, this force is specified even if it is only mentioned as "frictional force."
As shown in Fig. 3, the frictional force F is proportional to the vertical force component of the object and can be expressed as follows:
F＝μN
where µ is a constant of proportion determined by the load and load-bearing surfaces and is denoted the coefficient of friction. Then, how many kilograms of force will be required to pull an object weighing 1000 kg that rests on rollers on the ground (Fig. 2), if the coefficient of rolling friction is assumed to be from 0.001 to 0.003?
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Answer:
Plugging the following values into the above equation, µ = 0.001 to 0.003 N =1000 kg we get F = (0.001 to 0.003) X 1000 kg = 1 to 3 kg Therefore, the object will move if we apply to it a force from 1 to 3 kg
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The coefficient of friction in Fig. 1 is called the sliding coefficient of friction, and that in Fig. 2 the rolling coefficient of friction. Substituting a ball for the roller in the latter case yields the same result. Here, the roller and ball are called the rolling elements.
Normally, the sliding coefficient of friction is 10 to 20 times larger than the rolling coefficient of friction, and the rolling friction is almost the same as the so-called fluid contact, in which the object is buoyed by a fluid such as oil. However, because it is difficult to maintain this state for long, and because it is partially a sliding metal contact, the coefficient of friction µ is as large as 0.15 to 0.3 and fluctuates. For this reason, the rolling coefficient of friction is the more often used of the two coefficients.
So, since ancient times it has been known that heavy objects can be moved with relatively little effort if rolling elements are utilized under the load. However, it has been but very recently, about 100 years ago, that rolling elements such as bearings have been manufactured and marketed.
Why weren't rolling elements developed sooner?
For the same material, the larger the rolling element diameter, the harder it is to break; the smaller the diameter, the easier it is broken. This may be easier to understand if we use the analogy of a match, a pencil, and a log.
Accordingly, if the bearing is of wood or copper or other material known to the ancients, to support a given weight the rolling elements must be made larger. Thus, the bearing becomes so large as to be impractical, almost as large as the load itself. For this reason - lack of suitable materials - roller technology development was arrested.
Then came steel - but only somewhat over a century ago. Steel can be made very hard by heating it to very high temperatures and then abruptly quenching it. In this way, steel can be given very high crushing strength.
Accordingly, the load strength of a large-diameter wooden roller is possible with a very small diameter steel roller. Steel bearings found widespread use because the steel drastically downsized the roller and made it easier to use.
So, it is not exaggerating to say that the real history of the bearing began with steel.
The bearing has many advantages, as follows, in addition to light momentum, and is used according to its suitability to the application:
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(Single-Row, Negative Deep-Groove Bearing)
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(1) |
Large reduction in driving force |
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The loss of driving force is small, and heavy items are moveable by small forces because the area supporting the load moves smoothly. |
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(2) |
Small inertia |
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Very large forces are required to get heavy machinery moving. This is because the force required to overcome inertia (starting resistance) and get the load moving is about 3 times larger than that required to maintain the machine in motion once it is moving, whether friction is sliding or rolling. In fact, the starting resistance increases further in the case of a sliding bearing, because the lubricant is not well distributed due to its own weight when the machine stops, and because solid contact or semi-fluid contact occurs.
Compared with the sliding bearing, the rolling bearing is far smaller and requires less expenditure of power, and so is more economical.
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(3) |
Saves lubricants |
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The rolling bearing consumes a very small amount of lubricating oil in comparison with the sliding bearing, and it can further save expenses because grease can be used. |
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(4) |
Small abrasion |
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With the sliding bearing, abrasion cannot be avoided due to repetitive liquid and semi-liquid friction components. In contrast, the rolling bearing wears very little as long as foreign matter does not get into the bearing. |
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(5) |
Low maintenance cost |
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As the rolling bearing does not require daily inspection or lubrication, maintenance costs and maintenance are reduced. |
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(6) |
Improved product quality |
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As the bearing's rolling motion is inherently immune to abrasion as compared with the sliding bearing, machinery remains accurate longer, and thus product quality is assured. |
By the way, the first bearing, invented about 110 years ago, was a ball bearing using ball rolling elements. And, it was only about 40 years ago that the needle bearing, using thin, needle-shaped rolling elements, was invented. Its inventor was Hiroshi Teramachi, the founder of THK.
 Japanese motorbike manufacturers became world-famous about 1955, and their products suddenly received widespread attention when Honda won the Isle of Man championship.
Without a doubt, the contribution of needle bearings to the success of these products was very large. Needle bearings doubled engine speeds from 4000 to 8000 rpm, boosting horsepower 70% for the same engine displacement.
The motion of moving machine parts consists of a combination of rotational and linear motion. The above-mentioned bearings are those for rotational motion. In their short 100-year history, rotational motion bearings are now so widely spread that they are used in almost all rotational motion applications. Many bearings are used in automobile engines, and all around us there are many other invisible applications, such as in motors for video recorders, in roller skates, etc.
However, bearings for linear motion parts were relatively undeveloped or manufactured, though it was understood that there were many merits in these parts if they could be made with the rolling principle incorporated. Of the many types of mechanisms, linear motion parts are the most difficult to develop. Let us consider here why this is so.
If we enumerate the conditions required for linear motion parts, the following points are outstanding:
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(1) |
There are high rigidities in all directions, and the part moves lightly.
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(2) |
While moving lightly, positioning accuracy is easily obtained.
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(3) |
The overall cost is low.
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(4) |
Service life is long and accuracy is maintained for long periods.
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(5) |
Maintenance is easy.
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(6) |
Linear motion parts contribute to energy savings (economical).
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(7) |
Vertical and lateral accuracies are easily achieved.
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 These are necessary conditions, and it is extremely difficult for the linear motion part to satisfy all the listed conditions at once. Since almost all machines have the same requirements, these problems have long been subjects of linear motion development efforts. The "linear motion system" of our company was developed to solve these problems.
Thanks to this development, performance was largely improved for various mechatronics devices in Japan, such as numerically-controlled machine tools, semi-conductor manufacturing equipment, and industrial robots. This is yet another industry of Japan that has gained worldwide recognition.
Until recent years, industrial technology in Japan had been supported by introducing basic technology from Europe and America. Similarly, rotation bearings were first developed in the configurations known today by SKF of Sweden, a company that is still one of the most important bearing manufacturers in the world.
However, THK has striven, since the establishment of the company in April 1971, to develop a linear motion sliding method based on our own unique technology. We solved most of the difficult problems one by one, and one and a half years later, in 1973, we succeeded in developing a linear motion system that fully utilized the advantages of rolling in a linear motion bearing. We have since established a position as top manufacturer of linear motion systems, by developing and marketing new products to meet user needs.
Nevertheless, there remains a large undeveloped market, not limited to Japan, because it was not long ago that the linear motion system was developed.
As a stimulus to our goals of furthering development of this market and continuing to lead the field as top manufacturer of linear motion systems, we spell our name with the following spirited words:
T
for Toughness---We're the toughest contender
H
for High Quality--- We field the highest quality product
K
for Know-how--- When it comes to technology development.
we "know how" like nobody else
In other words, we are based on the management philosophy that "the product of the highest quality" is manufactured with the technical development capability to satisfy all users, and we have grown into a "tough enterprise" together with development addressing user needs. We pledge to always make rapid progress in these goals.
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