(* Spring parameters *)
R=50;l=400;n=7;r=4;

spring=ParametricPlot3D[
(R-r Cos[φ]){Cos[2 π n t],Sin[2 π n t],0}+{0,0,l t+r Sin[φ]} (* surface equation *)
,{φ,0,2π},{t,0,1},
Mesh->None, (* optional - only affects the preview below *)
PlotPoints->{5,50}, (* model mesh density *)
PlotStyle->White (* !IMPORTANT! - prevents model from being fragmented *)
]

(* Export *)
SetDirectory@NotebookDirectory[];
Export["Spring.obj",spring] (* additionally creates .mtl file - better delete it right after *)