Non-electrolytic zinc sheet coating – technical requirements and inspection of socket shoulder screw dimensions

The zinc-coated layer of the hexagon socket shoulder screw does not exceed the part (basic size) in the case of external threads. After coating, the ordinary thread is in accordance with the provisions of GB/T 3934: the external thread is inspected separately with the tolerance zone position h or H. When the coated thread is tested with a ring gauge, the maximum allowable torque is: 0.001 (N·m). Where d is the nominal diameter of the thread (mm). Other socket shoulder screw dimensions are only applicable before coating.

In order to achieve the specified corrosion resistance, if the minimum coating thickness is specified, the thickness range of the zinc coating layer shall be such that the socket shoulder screw dimensions is substantially equal to the minimum coating thickness. Therefore, the estimated maximum zinc coating thickness is twice the minimum required coating thickness. See Table 1. The minimum basic deviation of the thread applied for coatings with a minimum thickness of 4 or 8 is also given in Table 1.

Table 1 Basic deviation of coating thickness and requirements of non-electrolytic zinc sheet μm

Zinc sheet coating thickness		Minimum basic deviation required
Min	Max（Expected value）
4	8	32
5	10	40
6	12	48
8	16	64
9	18	72
10	20	80
12	24	96

Note: Since the thickness of the zinc sheet coating on the fastener support surface may cause slack, and reduce the clamping load, it is an important factor to consider.

If the basic deviation given in Table 2 for a given thread does not meet the minimum coating thickness required, then:

1. Change the tolerance band position of the thread (eg, by f instead of g)
2. Limit the tolerance to a given tolerance so that the resulting thread is the lower deviation of the external thread to its tolerance.

Table 2 Theoretical deviation of the thickness of ordinary threaded non-electrolytic zinc sheet coating  (μm)

PitchP/mm	Coarse thread nominal diameter d/mm	External Thread
		Tolerance zone position g		Tolerance zone position f		Tolerance zone position e
		Basic deviation	Zinc sheet coating thickness max	Basic deviation	Zinc sheet coating thickness max	Basic deviation	Zinc sheet coating thickness max
0.2		-17	4
0.25	1;1.2	-18	4
0.3	1.4	-18	4
0.35	1.5;1.8	-19	4	-34	8
0.4	2	-19	4	-34	8
0.45	2.5;2.2	-20	5	-35	8
0.5	3	-20	5	-36	9	-50	12
0.6	3.5	-21	5	-36	9	-53	13
0.7	4	-22	5	-38	9	-56	14
0.75	4.5	-22	5	-38	9	-56	14
0.8	5	-24	6	-38	9	-60	15
1	6;7	-26	6	-40	10	-60	15
1.25	8	-28	7	-42	10	-63	15
1.5	10	-32	8	-45	11	-67	16
1.75	12	-34	8	-48	12	-71	17

Note:

1. The theoretical deviation of the thickness of the zinc sheet coating is calculated based on the upper deviation value (external thread) of the specified thread tolerance.
2. The data providing the nominal diameter of the coarse thread is for convenience only. What determines the characteristic is the pitch.