HBS Blind Bolt

What is a blind bolt (HBS bolt)?

A blind bolt fixes a plate, bracket or beam to a hollow steel section when you can only reach one side. You cannot get a nut inside an SHS or RHS column, so the HBS bolt carries its own: a sleeve and a cone go in the hole with the bolt, and as you tighten the bolt head the cone is drawn into the sleeve and the sleeve expands behind the wall of the section, clamping the two together. Hobson lists it for fixing steel to square, rectangular and circular hollow sections, and for steel framework such as gantries, bridges, stadiums and transmission towers.

M8, M10 and M12 HBS bolts are 3-part assemblies (bolt, a sleeve with an integrated collar, and a cone). M16 and M20 are 5-part assemblies (bolt, collar, sleeve, cone and a rubber washer). On every size the cone has interlocking grooves that Hobson says prevent loosening, and you hold the collar (the setting nut on the Flush) with a spanner while you tighten the bolt.

Hex, countersunk or flush?

• •Hex head (galvanised class 8.8, M8 to M20, or 316 stainless with an A4-70 bolt, M10 to M20): a hex bolt head and a larger hex collar. Tighten with a torque wrench on the head while a ring or open-ended spanner holds the collar.
• •Countersunk, CSK (galvanised class 8.8, M8 to M16): a countersunk bolt with an internal hex drive and a collar with across flats for an open-ended spanner. Tighten with a hex drive bit in a torque wrench.
• •Flush (HBS-Bolt Flush, galvanised class 8.8, M8 to M12): the sleeve collar itself is countersunk, for a flush installation and a clean, flat finish. It needs a countersink in the fixture and the flush setting nut on this page to install; Hobson notes the collar has two slot channels to hold it in place during installation.

Choosing the length: grip range

Each HBS bolt length suits a grip range: the total thickness of steel being clamped, that is the fixture (plate, cleat or bracket) plus the wall of the hollow section. Add the two thicknesses and pick the length whose grip range takes that total.

The bolt diameter comes from the engineer's design; the grip only picks the length within that diameter. Worked examples on hex bolts: a 10mm cleat onto an SHS with a 4mm wall is a 14mm grip, so for an M12 design bolt the M12 x 60 (3 to 25mm) suits. A 16mm plate onto a 6mm wall is a 22mm grip: for an M10 design bolt that sits at the top of the M10 x 55 (3 to 22mm) and the bottom of the M10 x 70 (22 to 41mm), so measure the actual grip before you choose.

Minimum grip and fixture thickness differ by type. The 3-part M8 to M12 hex and CSK bolts start at a 3mm grip with a 1mm minimum fixture. The 5-part M16 and M20 bolts start at a 12mm grip with an 8mm minimum fixture. The HBS-Bolt Flush starts at an 8mm grip (M8), 9mm (M10) or 10mm (M12) and needs a minimum fixture of 5, 6 or 7mm.

HBS blind bolt sizes and grip ranges

Every HBS bolt we carry, with the grip range from Hobson's data sheets. Select a row to open that bolt in the selector.

Blind bolt capacities (from Hobson's data sheets)

Hobson publishes the same figures on its galvanised hex, CSK and Flush HBS data sheets, for each size. Read the conditions with them:

• •The characteristic loads are for an engineer to calculate design resistance. Hobson states they should not be used as working loads.
• •All the figures apply to the HBS bolt assembly only. Hobson notes the design resistance of the connection may be lower because of the other parts of the connection, such as the plate and the wall of the hollow section.
• •Hobson publishes these with the hole, spacing, edge distance, minimum fixture and torque values in the installation table below and gives no figures outside them. No combined tension and shear figures are given.
• •We publish no capacities for the 316 stainless HBS bolts: for a loaded 316 connection, the capacity has to come from the engineer's design.
• •A structural connection is designed by an engineer: use the size, number and layout of bolts on the drawings.

Hole size, spacing, edge distance and torque

Drill the hole to the size in the table through both the fixture and the hollow section. On Hobson's drawing, A is the centre to centre spacing between two HBS bolts, B is from the hole centre to the inside face of the section's side wall (room for the sleeve to open), and C is the edge distance from the hole centre to the outside of the section. F is the minimum fixture thickness.

Hobson's galvanised hex sheet adds a note for M8, M10 and M12: for maximum performance when the grip is within 2mm of the bottom of the bolt's grip range, use the minimum hole diameter.

How to install HBS blind bolts

Hobson's installation steps:

1. Bring the steelwork together, line up the pre-drilled holes and insert the HBS bolt through the fixture into the hollow section.
2. Hex head: have a ring or open-ended spanner for the collar and a torque wrench for the bolt head. CSK: an open-ended spanner, a torque wrench and an internal hex drive bit. Flush: an open-ended spanner, the flush setting nut and a torque wrench with an internal hex drive bit.
3. Put the spanner on the larger hex collar (on the Flush, on the setting nut) and the torque wrench on the bolt head or in its internal hex.
4. Hold the spanner still and tighten the bolt to the torque in the table. The sleeve expands behind the section wall as you tighten.

HBS-Bolt Flush setting nut

The HBS-Bolt Flush needs its setting nut to install: you hold the setting nut with an open-ended spanner while you tighten the bolt with a hex drive bit. Hobson includes one install tool in each carton of HBS-Bolt Flush bolts; these are extra setting nuts. They are for the Flush only: hex and CSK bolts have their own hex collar.

Galvanised or 316 stainless blind bolts?

Hobson rates hot dip galvanising as high corrosion resistance and recommends it for long term outdoor use. Its 316 HBS bolt is made for stainless steel applications, and Hobson describes it as highly corrosion resistant and suited to coastal applications.

Hobson's corrosion guide (written for metal connectors) sets the minimum by exposure: hot dip galvanising or stainless for steel exposed to the weather in most areas, stainless only in the sea spray zone, and 316 stainless within 2m of a pool edge. Inside the building envelope, galvanised is above the minimum. On a coastal job, match the bolt to the steelwork and the design life on the drawings.

Blind bolts in Adelaide

Allmat supplies HBS blind bolts, structural bolts, nuts and washers to fabricators, steel erectors and builders across Adelaide and South Australia. Add the bolts to a quote with the rest of your steelwork fixings, and we will price the lot.

Questions