Preload

Mechanical building terminology

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

The preload is Mechanical building etc. major A very common term. The general description is: in the connection (the connection mode and purpose are diverse), the force is applied in advance to enhance the reliability and tightness of the connection before the working load, so as to prevent the gap or relative slip between the connectors after the load.

Chinese name: Preload

Application occasions: Thread, bearing, etc

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Application occasions

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Preload

1、 Threaded connection In order to achieve the purpose of reliable fastening, it must be ensured that Thread pair With certain friction torque The friction torque is obtained by the pre tightening force generated by the thread pair after the tightening torque is applied during connection. The pre tightening force is related to the part material and thread diameter. For devices with pre tightening force requirements after connection, the pre tightening force (or tightening torque) data can be found in the assembly process documents. Special assembly tools can be used to control thread preload, such as force measuring wrench, torque wrench, electric wrench, pneumatic wrench, etc.

2、 Belt drive During installation, the belt is pre tensioned on the wheel, and the tension is called pre tension.

3. For bearings, a force, also called preload, has been applied to them in advance through static bolts, glands, etc. before use.

4. The spring preload is the maximum elastic recovery force and elastic time maintenance force considered in advance

5. In post tensioning method Prestressing process The concept of preload will be used in. stay Group anchor During construction, in order to improve the anchorage Efficiency, the number of cables to be tensioned at the same time shall be measured in advance steel strand Pre tension one by one, and then tension at the same time. During construction, only the total tension can be controlled. The purpose of pretension is to ensure that each single anchor in the same anchorage has the same tightness before tensioning, so as to play a role together in the working stage. The preload is generally small, about one tenth of the tension it should bear. The details shall be determined according to the duct length, duct friction, equipment travel, etc. during construction.

6. There is a popular saying among Chinese ancient builders: "tighten the car, rivet the dirty room, and press the table and stool by hand", which also means that the pre tightening force should be applied to the connection parts that are easy to loosen during work.

7. The windshield of a car is used rubber The rubber strip is H-shaped. The glass is clamped at one opening and the car body is clamped at the opposite opening. The clamp must be very tight. This is the preload. Tight clamping can ensure that the glass is installed stably and there is no water leakage at the clamping joint.^[1]

Correlation calculation

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Calculation of pre tightening torque of threaded connection

M t =K×P zero ×d×10 -3 kgf.m

K: Tightening force coefficient d: Nominal diameter of thread

P zero ： Pre tightening force (also refer to the table below) P zero =σ zero ×A s

A s =π×d s /4 d s : Calculated diameter of dangerous profile of threaded part

d s =（d two +d three ）/2 d three = d one - H/6 H: nominal working height of thread

σ zero =（0.5～0.7）σ s σ s―――― Yield limit of bolt material kgf/mm (related to strength grade, determined by material)^[2]

K value lookup table: (K value calculation formula is omitted)

Condition of friction surface	K value
Condition of friction surface	With lubrication	No lubrication
Finishing surface	zero point one zero	zero point one two
General machined surface	0.13~0.15	0.18~0.21
Surface oxidation	zero point two zero	zero point two four
Galvanized	zero point one eight	zero point two two
Dry rough machined surfaces		0.26~0.3

Pre tightening force P zero Lookup

Nominal diameter	Preload P zero （kgf）
	Strength level
	four point six	five point six	six point six	six point nine	eight point eight	ten point nine
M8	six hundred and ten	seven hundred and seventy	nine hundred and twenty	one thousand three hundred and eighty	one thousand six hundred and forty	two thousand and three hundred
M10	nine hundred and seventy	one thousand two hundred and twenty	one thousand four hundred and fifty	two thousand one hundred and ninety	two thousand five hundred and ninety	three thousand six hundred and fifty
M12	one thousand four hundred and ten	one thousand seven hundred and seventy	two thousand one hundred and ten	three thousand one hundred and eighty	three thousand seven hundred and sixty	five thousand and three hundred
M14	one thousand nine hundred and thirty	two thousand four hundred and ten	two thousand and nine hundred	four thousand three hundred and fifty	five thousand one hundred and twenty	seven thousand two hundred and twenty
M16	two thousand six hundred and thirty	three thousand and three hundred	three thousand nine hundred and fifty	five thousand nine hundred and twenty	seven thousand	nine thousand and nine hundred
M18	three thousand two hundred and ten	four thousand and thirty	four thousand eight hundred and twenty	seven thousand two hundred and forty	eight thousand five hundred and sixty	twelve thousand and one hundred
M20	four thousand and one hundred	five thousand one hundred and thirty	six thousand one hundred and seventy	nine thousand two hundred and thirty	eleven thousand	fifteen thousand and four hundred
M22	five thousand and ninety	six thousand three hundred and seventy	seven thousand and six hundred	eleven thousand and four hundred	thirteen thousand and four hundred	nineteen thousand and one hundred
M24	five thousand nine hundred and ten	seven thousand and four hundred	eight thousand and nine hundred	thirteen thousand and three hundred	fifteen thousand and eight hundred	twenty-two thousand and two hundred
M27	seven thousand six hundred and ninety	nine thousand and six hundred	eleven thousand and five hundred	seventeen thousand and three hundred	twenty thousand and five hundred	twenty-eight thousand and eight hundred
M30	nine thousand four hundred and ten	eleven thousand and eight hundred	fourteen thousand and one hundred	twenty-one thousand and two hundred	twenty-five thousand and one hundred	thirty-five thousand and five hundred
M36	thirteen thousand and seven hundred	seventeen thousand and one hundred	twenty thousand and five hundred	thirty thousand and eight hundred	thirty-six thousand and five hundred	fifty-one thousand and four hundred
M39	sixteen thousand and eight hundred	twenty thousand and five hundred	twenty-four thousand and five hundred	thirty-six thousand and eight hundred	forty-three thousand and six hundred	sixty-one thousand and three hundred
M42	eighteen thousand and eight hundred	twenty-three thousand and five hundred	twenty-one thousand and eight hundred	forty-two thousand and three hundred	fifty thousand	seventy thousand and eight hundred
M48	twenty-four thousand and six hundred	thirty thousand and eight hundred	thirty-seven thousand	fifty-five thousand and five hundred	sixty-five thousand and four hundred	ninety-two thousand and five hundred

influence factor

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The size of preload is not only limited by the strength of the screw material, but also limited by the material strength of the connected parts. When the material of internal and external threads is the same, only the strength of external threads can be checked. The strength of thread thread shall also be checked for the thread connection under axial load with short screw length, connection composed of non-standard threaded parts, and large difference in strength between internal and external thread materials. For example, the fixation of elastic elements of a certain type of product, because the base material for screw connection is die-casting aluminum alloy YL113, its strength is far lower than High quality carbon structural steel 20 should be checked thread The strength of the thread is mainly the shear stress and bending stress of the thread material.^[3]

Influence of preloading mode and speed

With the increase of rotating speed under constant pressure preloading Bearing The radial stiffness increases slightly, while the axial and angular stiffness decreases rapidly. Under the positioning preloading, the radial, axial and angular stiffness of the bearing increase rapidly with the increase of the rotating speed, but the increase of the axial and angular stiffness is relatively gentle. Ceramic ball bearing The stiffness change law of is similar to that of all steel bearings, but the change is relatively gentle. Under the positioning preload, the centrifugal force of the inner ring and the ball, as well as the effect of friction heat, increase the contact load of the inner and outer rings. At the same time, the contact angle of the outer ring decreases, and the contact angle of the inner ring increases, thus increasing the contact stiffness. However, the decrease of the contact angle of the outer ring slows the increase of the axial and angular stiffness. Under constant pressure preloading, the centrifugal force of the ball increases the contact load of the outer ring, while the contact angle decreases.

Since the inner and outer rings allow axial displacement, the inner ring contacts load Basically unchanged, but the contact angle increases. Thermal displacement and centrifugal displacement have little effect on contact load and contact angle of inner and outer rings. Although the outer ring is in normal contact rigidity Increase, but the normal contact stiffness of the inner ring is basically unchanged. The radial stiffness increases slightly as a result of the series action, while the axial and angular stiffness decreases significantly as the contact angle of the outer ring decreases.

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Under positioning preloading, the stiffness of ceramic ball bearings is less than that of all steel bearings, while under constant pressure preloading, the stiffness of ceramic ball bearings is greater than that of all steel bearings. Under positioning preloading, the contact load of all steel bearing is more than twice that of ceramic ball bearing, although ceramics The ball has high elastic modulus, and the stiffness of all steel bearing is greater than that of ceramic ball bearing. Under constant pressure preloading, the contact load of the inner ring changes little, and the high elastic modulus of the ceramic ball makes the stiffness of the ceramic ball bearing greater than that of the all steel bearing.

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Effect of preload

With the increase of preload, Bearing The radial, axial and angular stiffness of. Compared with positioning preloading, this effect is more significant for constant pressure preloading. This is due to the increase of preload, which increases the contact angle of inner and outer rings, and also increases the contact load, thus increasing the radial, axial and angular stiffness. However, the change of contact load and contact angle caused by preload is smaller than that caused by speed and part displacement rigidity Has limited impact. This is also the reason why the change under positioning preloading is less than that under constant pressure preloading.^[2]

Influence of channel curvature radius

With the increase of the radius of curvature of the inner and outer raceways, the radial, axial and angular stiffness decreases, but this effect is very small. Only the stiffness changes slightly under the positioning preloading, because the contact deformation increases with the increase of the radius of curvature of the raceways. Therefore, the influence of the channel curvature radius on the stiffness can be ignored when selecting the channel curvature radius 。

Influence of ball number

Under the positioning preload, the increase of the number of balls makes the radial axial And angular stiffness slightly increased. The increase of the number of balls will increase the stiffness, but under the same preload, the increase of the number of balls will reduce the contact load. Although their combined effect can increase the stiffness of the bearing, it is less.

Under constant pressure preloading, the increase of ball number makes the radial direction rigidity The axial stiffness and angular stiffness decrease when the speed increases to a certain value, but the change is small. This is because under constant pressure preloading, the increase in the number of balls reduces the contact load of the inner ring, but also reduces the contact angle of the inner ring. Their combined effect significantly increases the radial stiffness of the bearing, while the axial and angular stiffness slightly decreases.

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Therefore, when the number of balls increases, the preload should be increased accordingly. Only when the contact load is the same, increasing the number of balls can make Bearing Stiffness increases.

Influence of ball diameter

Under positioning preloading, the ball diameter increases, and the radial, axial and angular stiffness increases slightly. Increase the diameter of the ball centrifugal force The contact angle of the outer ring decreases with the increase of the contact angle of the inner ring, but at the same time, the contact load of the inner ring and the outer ring increases, and their combined effect increases the bearing stiffness. Contact is affected by the change of centrifugal force under positioning preloading load The influence of ball diameter change on stiffness is small.

Under constant pressure preloading, the radial stiffness increases with the increase of ball diameter, while the axial and angular stiffness decreases, but the impact is small. This is because the centrifugal force of the ball increases with the increase of the ball diameter, the contact angle of the inner and outer rings decreases, the contact load of the outer ring increases, and the inner ring contacts load Basically unchanged, so the radial stiffness increases, while the axial and angular stiffness decreases slightly. Therefore, reducing the ball diameter not only improves the speed performance, but also does not reduce the stiffness performance. This also proves that reducing the diameter of the ball is one of the development trends of the main shaft bearing.^[2]

Related changes

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Bearing stiffness changes with preload

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The trend follows Bearing With the increase of preload, the radial stiffness of the bearing becomes larger, which significantly improves the machining accuracy and working efficiency of the spindle system and improves the working performance of the spindle. Therefore, in the actual industry and mining, it is of great practical engineering significance to increase the preload within the allowable range. However, with the increase of preload, the bearing temperature As the temperature increases, the bearing heat generation will also increase, which will increase the temperature of the spindle system and seriously affect the working life of the bearing and the working performance of the spindle. Therefore, if the temperature rise is allowed, the main shaft is involved in increasing the preload as much as possible transmission An important factor to consider for the system.

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