Design of electric seat lifting system

Design of electric seat lifting system

Front lift of the seat: 30mm-50mm

Seat rear end lift: 30mm-45mm

Lifting load capacity of the front part of the seat: 120kg

Seat rear end lifting load capacity: 120kg

8.1 program selection

In the design of the upper and lower lifting system of the seat, a screw mechanism is selected, that is, the motor is decelerated by the worm gear, and then a mandrel is inserted into the worm, and the worm wheel drives the rise and fall of the mandrel to realize the adjustment of the up and down movement of the seat.

Features of the screw mechanism:

1) Since the nut can only be moved by one lead when the screw rotates one turn, and the lead can be made small, the screw mechanism can be used as the adjustment mechanism, where a large reduction ratio is required in the machine, and the screw mechanism is used. The mechanism makes it easier to simplify the drive train.

   2) Since the screw mechanism has a large reduction ratio, when a small torque is applied to the active member, a large thrust can be obtained on the driven member, that is, the screw mechanism has great mechanical advantage, and thus Suitable for use in jacks and presses.

3) Choosing a suitable helix angle can be a self-locking function of the screw mechanism. For example, the lifting screw of the workbench of the planer has self-locking property, so that the workbench can not be prevented from falling due to its own weight, and the lifting of the seat is more Need to be self-locking.

In addition, the screw mechanism has the advantages of simple structure, stable transmission and no noise. The shortcomings of the screw mechanism are mainly the low efficiency of the sliding screw, especially the efficiency of the self-locking screw mechanism is less than 50%.

8.2 Screw design calculation

The screw can be made of A5, A6 or 35-45 steel. When high quality carbon steel is used, heat treatment is needed to enhance the wear resistance of the thread. The screw is mostly trapezoidal thread.

When the screw is raised to the high position, the force is the most unfavorable. The screw is a pressure rod and is subjected to pressure and torque. Therefore, the thread is calculated according to the compressive strength, stability and self-locking conditions.

   8.2.1 Compressive strength calculation

When the screw is working, it is subjected to both pressure and torque. In order to simplify the calculation, only the effect of the pressure can be recorded, and the reduced compressive stress required is used. The compressive stress and strength conditions of the screw are:

Where: d1 - screw inside the mm

         Q - total axial force acting on the thread N

Because the seat lift load capacity is 120kg, the gravity acting on one of the two screws is 120/2*10=600N

   [σ]——Requires compressive stress, which can be 5~8KN/mm for carbon steel

   In the substitution type, available

D1 selects standard thread 8mm.

8.2.2 Calculation of stability

When the screw is at the highest position, it can be regarded as the lower end fixed, the upper end is free, and the diameter is d1. The checkpoint knows its end coefficient μ=2, and its stability can be checked by the following formula: nsc=Qc/Q≥ Ns

Where: scn——the calculation safety factor of screw stability

         Qc——the critical pressure of the screw

         Ns - design safety factor for screw stability

The design safety factor of screw stability ns can be 3~5, so ns=4, and the critical load Qc of the screw should be calculated by different formulas depending on the flexibility of the screw as λ=μL/ρ (where L is The calculated strength of the screw, taking the lifting height of the seat is 40mm, the radius of gyration of the cross section of the ρ screw, ρ=d1÷4=2mm, λ=2×40÷2=40

Because 40λ=less than 100, the material is 45 steel, the strength limit σ≥48k/cm2, take

Further, nsc = Qc / Q ≥ ns = 600, so the screw has good stability.

8.2.3 Accounting of self-locking ability

Because the self-locking condition is such that it has a reliable self-locking ability, at least the spiral angle of lift should be less than 1 degree of the equivalent friction angle. The equivalent friction coefficient fv=0.08~0.1., the corresponding equivalent friction angle should be

 8.2.4 parameters

    Check the Mechanical Design Handbook Table 3-8, the basic dimensions of the trapezoidal thread:

Nominal diameter d 8

Pitch p 2

External thread diameter d3 5.5

Inner diameter of inner/outer thread D2/d2 7

Internal thread diameter D4 8.5

Internal thread diameter d4 6

8.3 Height adjustment motor selection

8.3.1 Selection parameters

According to the requirements, the height of the seat is 40mm, the time required for the whole movement is 8s, and the lifting speed of the seat is v=s/t=5mm/s.

8.3.2 Torque of the screw

The axial force Q of the screw has been calculated previously, according to the formula:

8.3.3 Calculating the torque of the motor

M electricity *i* η=M wire

Here, the worm and worm speed reduction ratio is 62, and the transmission efficiency is 0.8.

M electricity = 119g × cm

Since the motor is a two-axis output DC motor of the sz series, the torque is:

M total = 2M electricity = 290g × cm = 0.290g × m

   For economical convenience, the motor with the same design as the system is selected to meet the required torque.

8.3.4 Checking the selected motor torque

According to the formula:

   Where: Td——output torque of electric shaft Nm

         T1 - the input torque of the working shaft, which is equal to the torque T on the turbine

   The formula is deformed as follows:

Through the above calculations, it is indicated that the selected motor is satisfactory, so the motor of the horizontal moving part is a permanent magnet type two-axis output DC motor of the 45sz01 type.

Check "China Machinery Design Industry Standard Compilation·Gear and Gear Transmission Volume (Bottom)" SWL Worm Gear Lift Form, Parameters and Dimensions, JB/T8809-1998. Select SWL2.5 based on motor speed, power, and lift speed.

The relevant parameters are as follows:

Maximum lifting capacity: 25KN

Maximum pulling force: 25KN

Worm gear ratio 24:1

Worm stroke per revolution: 0.250mm

Maximum elongation of the screw during tension load: 1500mm

Maximum allowable power: 0.55Kw

General ratio total efficiency: 23%

Lubricating oil quantity: 0.1kg

Weight without stroke: 7.3kg

Weight per worm of worm: 0.45kg

Note: 1. The maximum allowable power is the parameter under the condition that the ambient temperature is 20C° and the working duration is 20%/h.