The degree of coaxiality is one of the most important criteria to evaluate the quality of Universal testing machines.
As per the standards GB/T16491-2008 on Hydraulic Universal testing machines, the 0.5 degree machine max allowed error is 12%, and 1 degree accuracy machine max error is 15%.
2. Factors to effect the Coaxiality of Testing machines
2.1 Positional degree, verticality of mounting holes of upper and lower beam, baseplate, working platform
Coaxiality of Screw rod, column concentricity.
2.2 The accuracy of angles, flatness, symmetry between the beams wedge opening and Jaw splint.
3. The hazards of unqualified Coaxiality to Universal testing machine?
3.1 Easily to slip can't finish test
When testing the tensile strength of metal plate, it’s easily to occur slippage thus will result abnormal broken of samples, or can’t finish test. For example, the broken angle of aluminum board should be 45 degree, but if the coaxial degree not qualified, it can’t be 45 degree. When testing the tensile strength of rebar, it may have serious slippage thus will appear abnormal noise.
3.2 Can not conform to the test standards
Can’t adopt the strain rate of extensometer for close loop control because unqualified concentricity will result local plastic deformation or fracture happened within the gauge length of extensometer, under this condition can only adopt displacement rate of beam to for close loop control. Meanwhile, unqualified concentricity will seriously effect the measuring accuracy of Rp, Rt, Rr, Ae, Ag, Agt, At.