Question: System of differential equations

Hello! 

I have been using maple not so long ago and I can’t always understand for myself what is the reason for the program not working.
In this case, I need to solve a system of second-order differential equations with respect to four variables. I don’t understand what’s wrong, do not judge strictly.

// 
restart;

m1 := 50;
m2 := 70;
R1 := 0.14;
R2 := 0.17;
O1C1 := 0.3;
alpha := 0.024;
a := 0.28;
b := 0.08;
c := 0.08;
M0 := 12;
k1 := 1.7;
k2 := 0.9;
Zb := -a;
Md := M0 - k1*diff(varphi(t), t);
Mc := -k1*diff(varphi(t), t)^2;

Ixz := 0;
Iyz := m1*O1C1*(b + c) + m2*R2^2/4*alpha;
Izz := m1(R1^2/2 + O1C1^2) + m2*R2^2/4*(alpha^2 + 2);

eqMx := -M*Yc*diff(varphi(t), t $ 2) - M*Xc*diff(varphi(t), t)^2 = Xa + Xb;
eqMy := M*Xc*diff(varphi(t), t $ 2) - M*Yc*diff(varphi(t), t)^2 = Ya + Yb;
eqIy := -Ixz*diff(varphi(t), t $ 2) + Iyz*diff(varphi(t), t)^2 = Zb*Yb;
eqIx := -Iyz*diff(varphi(t), t $ 2) - Ixz*diff(varphi(t), t)^2 = Zb*Xb;
eqMz := Izz*diff(varphi(t), t $ 2) = Md + Mc;
sys := {eqIx, eqIy, eqMx, eqMy};
dsolve({sys, varphi(0) = 0}, Xa, Xb, Ya, Yb);

Error, (in dsolve) invalid arguments; expected an equation, or a set or list of them, received: { and so on }

//

I will be glad if you help me with this problem. if there are certain questions, you - ask, and I'll answer.
Here you can find the file: Курсовой_по_термеху_2.mw