This mechanism was invented by engineers for lifting loads of any mass. Lifting mechanisms of this type are divided into:
Screw jack is characterized by mobility and compactness, however, it can lift the weight of the load up to 50 tons.
The main advantages of the jack are its compactness and mobility. The design of the jack far exceeds the compactness of winches and other lifting mechanisms. In addition, the jack can be installed directly under the load, so no additional devices are required to lift the product. Such excellent properties allowed the use of a jack in the construction industry and when it is necessary to perform installation work. Every motorist necessarily has such a mechanism in his trunk. You can make the jack and do it yourself, but for this you need to perform the calculation of the screw jack.
How is the screw mechanism?
The design of such a device is composed of:
The device of the screw jack.
- bronze nut;
- a screw having a rectangular thread (sometimes it is made trapezoidal);
- cargo heads;
A wheel equipped with a doggie, endowed with teeth and fixed on a dedicated square of the shaft axis, is called a ratchet. According to the direction of the screw such a dog has the ability to turn on the shaft, occupying an extreme position. From turning it keeps the stopper with a spring fixed on it.
Such a jack belongs to the group of self-braking mechanisms. For such a transmission is characterized by a small lifting angle of the screw, denoted by the letter "k". Its value is always less than the angle of friction, denoted - "p".
I must say that this angle is considered a negative property of this type of transmission. The efficiency of the mechanism does not reach 0.5. At the same time, the screw jack is capable of lifting a weight of 50 tons to a height of about 35 cm. When the weight exceeds 20 tons, a much greater force must be applied to the handle. In this regard, the ratchet has to be changed with a more powerful worm gear. The jack is equipped with a machine drive.Back to table of contents
Basic formulas used
The overall calculation of the jack is made according to a certain formula: P = Q * z * tg (a + p) / R + n, where:
- P - the force that is applied to the handle when the screw rotates by hand;
- Q - applied force;
- R is the size of the handle;
- n - efficiency.
The great advantage of the screw jack is the possibility of self-braking. The load will not be able to lower until the rotation of the screw begins.
Effort "Q", which is able to develop a jack, depends on the load "P", which is required to give the lever. Mathematically, this calculation looks like this: Q = P * (2R * L / t) * n, where:
- Q - the force that develops the jack;
- P - pressure on the handle;
- L is the size of the lever;
- t - threaded pitch;
- n - efficiency.
Efficiency is calculated by the following formula: n = tga / (tg (a + p)), where:
- and - the angle of ascent, which has the ability to cut a thread;
- p is the angle of friction;
When the screw moves upwards, the work of the jack is carried out in compression, depending on the mass of the load, on how much friction appears.
The resulting compressive stress, when there is a powerful axial load, can be calculated using the following formula: Cf = Q / (R * d / 4), where:
- Q is the mass of the load;
- d is the inner diameter of the axis.
To calculate the torsional stress, which forms the torque, the following expression is used: Lcr = Mcr / Wcr, where:
- Wкр - the moment resisting to torsion;
- Wcr = (Rd / 16) = 0.2 * d.
Lifting capacity of the screw jack is calculated by the formula.
To calculate the torque, do the following calculations: Mc = Q * tg (a + p) * (dav / 2), where dav is the average diameter of the axis.
The equivalent voltage appearing in the axis is given by the formula: Q = (2Qсж + 3n) / 2.
The screw thread, cargo jacks is a = 4. The magnitude of the friction angle is as follows: p = 6. Average diameter - LLd.
When the transformations are completed, the equation takes the following form: Qсж = 1.25Q / ((Rd / 4) * f), where f is the slope coefficient (selected according to the table according to the flexibility of the screw).
The calculation of the jack is carried out only in compression. The main factor is the weight of the cargo "Q". The calculated twisting coefficient is taken equal to - 1.25.
When it becomes necessary to calculate the outer diameter of the screw, a specific formula is applied: d = d1 + 2a, where:
- a is the thickness of the thread (in most cases it is calculated as follows: a = t / 4);
- t - threaded pitch.
Approximately: d = 1.2-1.25 * d1.
The specific pressure of one coil is: q = Q / F1 = Q / (R / 4 (d2-d1) z (N / mm), where:
- F1 is the area of contact of the screw nut;
- z - the number of turns, which has a nut thread.
Self-made screw jacks are made of coupling screws and nuts.
The number of turns is calculated by the mathematical formula: z = Q / (R / 4 (d1-d2) q.
The strength of the turns from the shearing effect on the nut is calculated as follows: Q / Rd1 * b * z, where:
- b - the width of the threaded coil;
- b = t / 2 mm.
The number of turns "z" is calculated using both formulas. In the subsequent calculation, the largest number is selected.
The thickness of the nut is: H = zt (t is the thread pitch).
To calculate the dimensions of the screw, starting from the strength, you need to know the maximum lifting height.
It is very important to take into account in this case the coefficient of stability - f.
The stability of the screw can be checked using the formula: Oсж = Q / f * F1, where F1 is the cross-sectional area.
When the calculation of the flexibility of the screw is done, the required length is required to be considered double, with respect to the extended part.
The operation of using screw jacks and their varieties requires the implementation of certain rules. That is, the installation of the jack must be done strictly under the center of the load. This is due to the fact that the screw jack works in compression. Therefore, if an incorrect installation is performed, it will begin to bend.
It is impossible to make a longer jack handle, since a large application force can break the screw. Its cross section will not stand.Back to table of contents
The efficiency of the jacking jack is 0.3-0.4, its carrying capacity is up to 50t., The height of lifting the load can be 0.4-0.6 m.
Unlike the screw hydraulic jack works on special liquid. The principle of operation of the hydraulic jack is laid in the work of the drive pump. Hydraulics and its laws are the basis for such work. The pump creates a certain pressure of hydraulic fluid, which moves the plunger. As a result, the load begins to rise. Then the liquid goes into the pump tank, the load is lowered.
- large load capacity;
- minimum labor costs;
- high efficiency;
- mild stroke;
- rigid construction;
- lift parts too high;
- it is not possible to adjust the lowering height;
- often break down, repairs are very complicated and expensive.
The lifting capacity of the hydraulic jack reaches 200 tons. The jacks are divided into several types:
- bottle (single and telescopic);
Yes, everything is complicated and simple at the same time. You can get confused. However, take the minimum of time, turn on the logic, and there will be no problems with the calculation of the jack. Good luck!