Определение передаточных чисел раздаточной коробки

    В раздаточных коробках полноприводных машин имеются, как правило, две ступени: низшая и высшая. Высшая является прямой или имеет передаточное число близкое к единице (0,95…1,05). Передаточное число низшей передачи в раздаточной коробке определяется из тех же условий, что и передаточное число на низшей передаче в коробке передач, т.е. исходя из условий:

 

1) преодоления транспортной машиной максимального подъёма:

                        и_{РКн ψ} ≥    .

 

2) полного использования сцепного веса транспортной машины:

                        и_{РКн φ} ≤   .

 

3) обеспечения минимальной устойчивой скорости движения:

                        и_{РКн v} = 0,377 ·  .

 

     При расчётах принимают: ψ _max = 0,7…0,9; = 0,8…0,9;                   V _{тм min} = 2…3 км/ч.  

 

Оценка тягово-скоростных свойств транспортной машины

Уравнение движения ТМ

    Оценку тягово-скоростных свойств ТМ производят, решая уравнение её движения. Уравнение движения ТМ связывает движущую её силу, с силами сопротивления и позволяет определить характер прямолинейного движения ТМ, т. е. в каждый момент времени найти ускорение, скорость, время движения и пройденный транспортной машиной путь.

       Окружная сила на ведущих колесах F_к при движении ТМ затрачивается                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         на преодоление сил сопротивления воздуха F _в, качению F_f, подъему F _h и разгону F_а, т. е.

                               F _к – F _в – F_f ± F_h ± F _а = 0.                  

     Здесь знак “–” при силе F_h соответствует движению ТМ на подъеме, а знак “+” – движению на спуске; знак “–” при силе F _а соответствует разгону автомобиля, а знак “+” – торможению.

    Записав в этом уравнении вместо сил выражения, их определяющие, полу- чим:

 

Разрешив последнее выражение относительно ускорения ТМ, для случая её разгона будем иметь:         

Данное уравнение называется уравнением движения транспортной машины.

Здесь:	mтм	– полная масса ТМ, кг;
	Gтм	– сила тяжести ТМ, Н;
	ro	– расчетный радиус качения ведущих колес, м;
	u тр	– передаточное число трансмиссии ТМ;
	h тр	– коэффициент полезного действия трансмиссии;
	kв	– коэффициент сопротивления воздуха, Н с^2/м^4;
	Vтм	– скорость движения ТМ, м/с;
	Aв	– площадь лобового сопротивления ТМ, м^2;
	f	– коэффициент сопротивления качению;
	i	– коэффициент сопротивления подъему (тангенс угла наклона дороги по отношению к горизонту);
	a	– угол наклона продольного профиля дороги, град(рад);
	δ	– коэффициент учета вращающихся масс ТМ.

Это уравнение справедливо для неустановившегося движения ТМ.

                    δ = 1+                  

Решение уравнения движения ТМ в общем виде аналитическими методами практически невозможно, так как неизвестны точные функциональные зависимости, связывающие силы, действующие на ТМ, с её скоростью. Поэтому уравнение движения ТМ решают численными методами на ЭВМ или приближенно, используя графо-аналитические методы. Наибольшее распро-странение получили метод силового (тягового) баланса, метод мощностного баланса и метод динамической характеристики.