Wheelchair Size and Material Application in Human-machine System Model

(1) Cushion can be said with user's most personal place, and its design will be in accordance with user's hip size and shape of curve design. Saddle load after magnitude of pressure should not be too deep, that is, to support force enough, otherwise ride will sit uncomfortable.

(2) Backrest is divided into adjustable and fixed-angle two categories. Back design is generally based on human back curve, comfort and safety principles.

(3) Handle can be divided into directional and bi-directional, two-way handle because it can be commuted to implementation facing occupants. There are also handle can adjust height, which is designed for different height. The use of other people attention does not hang on handle of goods, so as not to focus instability and flip, injury to occupants.

(4) Wheelchair frame of iron pipe usually heavier weight, but has good stiffness and strength assurance; aluminum tube light weight, but it's not as good as iron and steel safety assurance. New materials, synthesis of aluminum alloy can solve this problem, in light and convenient at same time have a good security guarantee.

(5) In the face of less flat road, the soft cushion has been unable to provide reliable comfort, so general wheelchair are equipped with anti-shock function, in order to bumpy road, shockproof devices are generally installed front wheel combination or rear wheel group. while selecting, car can be placed on the ground, light compression frame test period of elasticity.

(6) Brake device has a deceleration function, when wheelchair parked on slope of terrain, operator will not hold frame, and wheelchair will slide at any time and overturned, easy to cause great risk to occupants; The brake is one of necessary safety equipment for wheelchairs.

(7) Drive is an indispensable part of wheelchair, and its design is usually convenient and flexible, labor-saving security principle.

Model dimensions of wheelchair are based on related theory of ergonomics. In design process, data applications of ergonomics are basically through human-machine database to retrieve, and calculate the data [5].

2.2.1

Human Sitting Body Size Selection

Sizes of human body structure are vastly different, and their activities are irregular, but seat is of a fixed size and movement regularly. This design is necessary for structure of human body size by taking mathematical statistics method, and gaining regular size from probability [6]. Fig. 1 [7] and Table 1 [7] are design-related basic data of human sitting body size in national standard GB/T 10000. Numbers in Figure 1 correspond to sequence number in Table 1. Fig. 2 [7] and Table 2 [7] are design-related basic data of human level body size in national standard. Numbers in Fig. 2 correspond to sequence number in Table 2.

Fig. 1

Fig. 2

Table 1

Sitting body dimension(part). (unit: mm)

Age groups	Male (age of 18–60)							Male (age of 18–55)
Percentiles
Measurements
	1	5	10	50	90	95	99	1	5	10	50	90	95	99
3.8 Leg and foot height	372	383	398	413	439	448	463	331	342	350	382	399	405	417
3.9 Sitting depth	407	421	429	457	486	495	510	388	401	408	433	461	469	485

Table 2

Horizontal dimension of human body(part). (unit: mm)

Age groups	Male (age of 18–60)							Male (age of 18–55)
Percentiles
Measurements
	1	5	10	50	90	95	99	1	5	10	50	90	95	99
4.6 Sitting hip width	284	295	300	321	347	355	369	295	310	318	344	374	382	400

Wheelchair design is object, which it is clear that extremely inconvenient for the elderly and the disabled to make them comfortable is necessarily considered. From perspective of human physiology, thoracic spine is bent outward and spine of waist is curved inwardly. From point of view of biomechanics, human body to withstand weight of upper body and sitting human body, so pressure on the chair surface should be in accordance with different parts of buttocks to bear the principle of different pressures to design, body posture hip body pressure distribution curve shown in Fig. 3 [7]. The figure shows, subject from upper body and leg pressure is largest ischial tuberosity, marked with location of number 90. Due to longtime sitting posture, this joint tends to hurt. Therefore, chair surface and backrest shape, height and thickness of design surface structure should be in line with shape of human hip curve, mainly scattered ischial tuberosity of force to reduce degree of fatigue and sitting to prevent occurrence of related disease, in line with human body distribution curve.

Fig. 3

As people in process of dressing activities, also to meet people's psychological needs of all for beauty and comfort, design size should be in addition to body size and percentile according to above sitting position choice, but also need to add appropriate correction of wear and psychological correction amount. Best function of product Size = body size percentile + function correction + mental correction.

Wheelchair is for the disabled, the elderly ride in order to achieve purpose of assisted mobility, and the elderly and the disabled themselves is very hard to use, so safety must be considered most important design, any potential hidden safety dangers are likely to hurt the occupants, so in structural design, we must take into account safety.

In recent years, new green materials continue to heat up, began to occupy a new market areas. Along with global economy of green economy, the market heat goes rising, meanwhile new environmental trend and new materials is sweeping the world. Green wheelchair design boom has also led to a huge demand for green materials. Green materials as a wheelchair manufacturing base and source, more and more become mainstream of market procurement. Green materials market will not only bring about changes in consumption patterns, but also a new round of changes of furniture industry.

Axial tensile and compressive deformation of lever is in its simplest form. Up in machinery and instruments, subjected to tension and compression rod role in process of folding wheelchair, coupling often used bolts are tightened after bolts to withstand tension. The forces of lever by analysis of axial tensile or compressive axial action shows that lever tension and compression force characteristics of role of force in two ends of rod member size is equal and opposite in direction, and action line of rod coincide with axial member. Deformation characteristics of a rod member axially stretched or shrunk, while its cross section becomes thinner or fatter [8]. This is shown in Fig. 4.

Fig. 4

From foregoing, in tension or compression, internal force rods have cross section along axis of rod, this force is called axial force, and axial force can be tension, as pressure. Internal forces division tension member cross section to determine stress points of cross section, lever under action of external forces not only to generate internal forces, but also deformed, and between internal forces and deformation are closely related. So you can experiment to observe deformation, to understand distribution of internal forces [9].

First take uniform bar on its surface draw two horizontal line perpendicular to rod axis ab and cd, and between two horizontal lines drawn parallel to longitudinal axis of rod line. Then rod ends with a pair of axially pulling rod member tensile deformation. From rod surface can be observed: ab cd straight and flat, respectively, moved a1b1 and c1d1 position, and rod remains straight and perpendicular to axis; two longitudinal lines stretch and elongation equal and remains parallel to axis of rod. According to observed surface phenomena, may make assumption of plane: Before deformation of cross-sectional plane, still plane after deformation, but translation occurs along axis. This is shown in Fig. 5.

Fig. 5

According to assumption of plane found, any cross-section of each longitudinal line between two elongation (or shortening) are same. Uniform continuity assumption by material shows that cross section of internal forces are uniformly distributed, ie each point equal stress.

Let rod cross-sectional area be A, cross section of axial force N, normal stress of cross-section of σ = N/A (sign of σ is same axial force, when N is positive, it is also positive, called tensile stress; when N is negative, and it is negative known as stress). From above we can see that rod material can withstand stress is limited, to ensure normal operation of lever, lever operating stress must not exceed allowable stress of material. Therefore, stretched conditions of rod for compression strength are (1) σ=FNA≤[σ](MPa) \sigma = {{FN} \over A} \le [\sigma ](M{P_a})

Where [σ] is allowable stress of rod material, MP_a. Dsign of main structure will be known in a wheelchair with a diameter of 2CM pipe, assumptions required by 90KG weight, namely sectional area is A=π4d2=314 (mm2) A = {\pi \over 4}{d^2} = 314\;( {{\rm{m}}{{\rm{m}}^2}} ) . According to stretched or compressed rod strength formula as follows (2) σ=FNA≤[σ](MPa) \sigma = {{FN} \over A} \le [\sigma ](M{P_a}) it can be gained σ=FNA=28.7MPa \sigma = {{{F_N}} \over A} = 28.7MPa

Currently on market use a wheelchair most of hard material aluminum alloy tube, maximum tensile strength of 370 MP_a, maximum allowable stress [σ]370 MPa, and result was far less than calculated maximum allowable stress, so main structure wheelchair with hard aluminum.

Rod under effect of different loads will produce different variants. Depending on nature and location of loads itself, deformation can be divided into axial tension (compression), shear, torsion, bending four basic deformations. Axial tensile and compression is not described in detail in this section, following analysis will focus on torsion and bending theory of selection in a wheelchair.

Circular shaft torsional stiffness condition is that maximum circle axis unit length does not exceed allowable twist angle with a twist angle per unit lengt [θ], namely θmax=TGIP≤[θ](rad) {\theta _{\max }} = {T \over {G{I_P}}} \le [\theta ](rad) or θmax=1000TGIP×180π≤[θ][(∘)/m] {\theta _{\max }} = {{1000T} \over {G{I_P}}} \times {{180} \over \pi } \le [\theta {][(^ \circ })/{\rm{m}}] .

Beam bending stiffness condition is: at the time of beam bending deformation, deflection and rotation of a designated section must not exceed allowable value, namely y_max ≤ [y] or y_max ≤ [y].

Where, [y] and [θ] are allowable deflection and allowable angle, both of which can be found in relevant design manual.

3.3.1

Calculation of torsion rigidity

If each role by an external even force Me at both ends of a straight bar, and both have same size, rotate in opposite directions perpendicular to axis of lever and action face, cross-section of lever about axis of relative rotation, this torsional deformation is called [10], as shown in Fig. 6.

Fig. 6

Deformation features are: any two cross-section rods are rotated around axis of rod relative to relative angular displacement between two cross-section called twist angle, presented by φ, which is ection B with respect to section A of twist angle. Longitudinal torsion of bar lines occurs slight tilt, and tilt angle of surface with a longitudinal line representation, namely γ. As shown in Fig. 7.

Fig. 7

Known twist angle per unit length of used hard aluminum tube is [θ] = 1(°)/m, and shear modulus is G = 8 × 10⁴MP_a.

According to above fomulas, θmax=1000TGIP×180π≤[θ][(∘)/m] {\theta _{\max }} = {{1000T} \over {G{I_P}}} \times {{180} \over \pi } \le [\theta {][(^ \circ })/{\rm{m}}] and IP=πd432(1-α4)≈0.1d4(1-α4) (mm4) {I_P} = {{\pi {{\rm{d}}^4}} \over {32}}(1 - {\alpha ^4}) \approx 0.1{d^4}(1 - {\alpha ^4})\,{\rm{mm}}^4} , it can be gotten that θmax=1000TGIP×180π=0.73(∘)/m<[θ]=1(∘)/m {\theta _{max}} = {{1000T} \over {GIP}} \times {{180} \over \pi }{ = 0.73(^ \circ })/{\rm{m < [}}\theta {\rm{] = 1}}{{\rm{(}}^ \circ }{\rm{)/m}} .

Therefore, rigidity of hard aluminum alloy tubing is enough.

3.3.2

Calculation of bending stiffness

If ends of each straight bar by effect of a coupling force Me, and both are equal in magnitude, opposite turning, acting surface are coincident with a plane including longitudinal axis of rod, or be located in longitudinal plane and perpendicular to rod when external force F acts axis, axis of rod member would bend, this deformation is called bent, as shown in Fig. 8 (a) and (b). Fig. 8 (a) shows pure bending, and Fig. 8 (b) shows a cross bending force [11].

Fig. 8

Draw main structural support beam shear and bending moment diagram, final structure of shear and bending moment diagram chart analysis [12], which is shown in Fig. 9.

Fig. 9

Allowable deflection of beam bending is [y], and it is enough as long as allowable deflection of chosen material does not exceed maximum deflection. Maximum deflection of main structure beam is (3) ymax=IZWZ {y_{max}} = {{{I_Z}} \over {{W_Z}}}

(Where I_Z – inertia moment, mm⁴, W_Z – b ending section modulus, mm³) And because material used is a hollow aluminum pipe, pipe cross-section inertia moment IZ=πd464(1-α4) {I_Z} = {{\pi {d^4}} \over {64}}(1 - {\alpha ^4}) and bending cross-sectional model WZ=πd332(1-α4) {W_Z} = {{\pi {d^3}} \over {32}}(1 - {\alpha ^4}) separately exist. That means according to conditions of bending stiffness, ymax=IzWZ {y_{max}} = {{{I_z}} \over {{W_Z}}} , it is enough as long as allowable deflection of chosen material does not exceed maximum deflection [y]. Through experiments and analysis, use of duralumin design of wheelchair entirely meet conditions and sufficient rigidity, so use of aluminum as main structural material is enough to meet requirements.