Abstract This article discusses the allowable lifting capacity of forklift trucks after being equipped with attachments and the overload protection problems of forklift trucks based on the stability of forklift trucks, component strength, safety and practical application in work.
Keywords: Forklift truck Attachment Lifting capacity Overload protection
Forklift is designed according to standard fork and rated lifting weight. Because the attachment is configured after the forklift is made and the weight is generally larger than the original fork, the load center is also different, causing the center of gravity of the entire forklift system to change. In order to ensure the safety and reliability of the forklift, without changing the structure of the forklift body and various safety indicators, the allowable lifting weight of the forklift should be re-determined after the attachment is installed. At present, the types of forklift attachments are constantly increasing. It is very important to correctly determine the allowable lifting weight of forklifts after they are equipped with attachments. This is of great significance for ensuring the safe use of forklifts and making full use of the lifting capacity of forklifts.
1 Determine the allowable lifting weight according to the stability principle
The load on the front and back of the forklift before and after changing the attachment should not exceed the allowable value, that is, the maximum load torque of each component remains unchanged. At this time, the allowable lifting weight after using the attachment can be determined through stability calculation. Based on the center of the front wheel of the forklift, the torque of the forklift's rated lifting weight and the weight of the removed standard fork to the center of the front axle is equal to the torque of the new allowed lifting weight and the weight of the attached attachment to the same center. (See Figure 1) Available:
In the formula, Q——rated lifting weight
Sc——load center distance
A——Fork weight
Gc——Fork weight
Gcx——The weight of the new attachment
Sbc——The horizontal distance from the center of gravity of the new attachment to the vertical front surface
Sbx——The horizontal distance from the center of gravity of the new attachment to the front surface of the fork
Z——the horizontal distance from the rear surface of the vertical section of the fork to the front axle
Scx——the horizontal distance from the load center specified for the new attachment to the front surface of the vertical section of the original fork
Qx——Allow lifting weight when configuring attachments
Equation (1-2) can be obtained after the forklift is equipped with attachments and allowed to lift weight.
The forklift works under this allowable lifting weight, and will not cause damage to the performance of the whole machine. It is the same as the original standard fork. This method can be used as a basic principle for determining the allowable lifting weight after the forklift is equipped with attachments.
2 Determine the allowable lifting weight according to the load bearing principle of components
In the first article, it can be derived from formula (1-2): if the new load center distance Scx is greater than the original load center distance Sc, the new allowable weight can be determined by this formula. If the load center distance is reduced and Scx is less than Sc, the new allowable lifting weight obtained using equation (1-2) may be greater than the original rated lifting weight. This situation is absolutely not allowed in the actual operation of the forklift. Because the allowable safety factor of the hydraulic system such as the door frame components, the lifting chain, the lifting hydraulic cylinder, the bearing part of the drive axle and the multi-way valve should not be changed. In the overall design of the forklift, each component only constitutes a safety guarantee for the forklift under the rated load. Therefore, under this condition, the new allowable lifting weight can be determined according to the principle that the forklift and the components should bear the same load before and after the attachment. You can get the following formula:
In the above formula, Qy is the new allowable lifting weight after the attachment of the forklift is determined by this principle.
The forklift can still maintain its original performance under the allowed lifting capacity. This method determines the necessary principle of the allowed lifting capacity after the forklift is equipped with attachments.
3 Comprehensive analysis
According to formula (1-2) and formula (2-2), we can roughly draw the relationship graphs of Q, Qx, Qy and Sc, Scx, see Figure 2. In the figure, Q curve is the load curve when the fork is used; Qx is the new allowable load curve after the forklift is equipped with attachments calculated according to formula (1-2); Qy is according to formula (2-2) New calculated allowable load curve.
The shaded part where each curve intersects below is the safety zone, that is, the allowable load range of the forklift equipped with attachments.
According to the figure, the following situations can be drawn:
(1) When Scx>Sc, use formula (1-2) to calculate the new allowable lifting weight, which is the most restrictive, because the resulting Qx value is the smallest and the safety factor is the best.
(2) When Scx>Sc, use formula (2-2) to calculate the new allowable lifting weight, which is the most restrictive, because the Qy value obtained by it has the best insurance value for the forklift's working operation, it will not Damage caused by forklift.
(3) When Scx is close to Sc, due to the greater influence of fork self-weight Gc and attachment self-weight Gcx, it can be calculated by formula (1-2) and formula (2-2) respectively, and then take the small value as the allowable starting weight.
4 Overload protection of forklift under different load center distance
From the foregoing analysis, it can be seen that the same cargo, when the center of gravity is at a different position in the length direction of the fork, the impact on the load capacity of the forklift is different. When the weight of the cargo is unknown, lifting it with a forklift truck may cause overload. There are three cases to discuss below.
Assume that the weight of the cargo is greater than the rated lifting weight of the forklift.
(1) The horizontal distance between the center of gravity of the cargo and the vertical surface of the fork is less than the center distance of the forklift load.
At this time, according to equation (2-2) and Figure 2, the lifting capacity of the forklift is: Q1=Q=Qy (2) The horizontal distance of the center of gravity of the cargo from the front surface of the vertical section of the fork is close to or approximate to the forklift load center distance.
At this time, Scx=Sc, according to formula (1-2), formula (2-2) and Figure 2, the lifting capacity of the forklift is: Q2=Q=Qx (3) The center of gravity of the cargo is away from the front surface of the vertical section of the fork The horizontal distance is greater than the forklift load center distance.
At this time, Scx>Sc, from formula (1-2) and Figure 2, we can see that the lifting capacity of the forklift is: Q3=Qx The forklift safety operation regulations are strictly restricted, and it is not allowed to place the known overweight cargo beyond the load center Forced lifting operation from the position. Only in this way can the forklift get effective overload protection.
