Sean Keeley, managing director of supplier Rivetwise, says: “People underestimate riveting. They just think it’s a simple deformation process, and they see the rivet tools are easy to pick up and put down. As good as engineers might be, they can be naive about riveting.”
He says this with valid cause: “We just had a large automotive manufacturer contact us where an incorrect nosepiece was used for a Monobolt-type rivet on 350 vehicles, and it is going to have to drill them all out. It is so important that the equipment is spot-on for a rivet that is being used, or it can go horrendously wrong.”
A Monobolt rivet has a ring on the face of the nose that helps lock the retaining part of the mandrel in the body so it doesn’t rattle. But if you install a Monobolt rivet with a standard nose, the rivet will pull through and render it useless.
In their case it is possible to remove Monobolt rivets, but it’s a tricky business. “You have to be so careful to nip the head off and hope that the rest drops out the blind side. You don’t want the drill to go through and expand the hole, because then it will be too big for the rivet, and a new set of rivets won’t fit.” In case the worst happens, Rivetwise supplies a rivet removal tool that fixes the depth of the drill bit. (Alternatively, collar splitters are recommended by Richard Avery, sales director of Zygology).
Keeley explains that Monobolts are a type of structural rivets, which have a more robust internal construction to offer greater performance, though they come in the same sizes as standard pop rivets. As he alluded above, they require specialist nose equipment [for the rivet gun], although this is readily available. Keeley adds: “That’s what we do: make sure you have the right rivet, for the right application, with the right installation accessory. It becomes a whole system, and that’s what we specialise in – matching it all up.”
Preventing tooling mismatches involves either changing the fastener to prevent the possibility of confusion, or colour-coding parts and introducing assembly line process controls.
While the devil may be in the detail, the big picture is that structural and standard blind (or pop) rivets work in exactly the same way. These small, two-piece, permanent mechanical fasteners resemble a toy sword, with a thicker ‘handle’ section about a third of the length and a thinner mandrel section making up the rest, with a flared collar in the middle. The thicker ‘handle’ section is in fact a tube around the mandrel that includes the collar. Users insert the thick end of the rivet into the hole with the mandrel projecting backward. A 6mm hole depth might require a 10mm long rivet.
Then workers use a tool – manual, pneumatic or battery-powered – to pull on the mandrel. Because the mandrel is slightly wider than the outer ‘handle’, as it passes through, it permanently deforms that section against the rear surface. After that, the mandrel shaft breaks off at a pre-weakened point.
Adds Avery: “When you talk about riveting, it’s nearly always in blind holes, so operators can work on an assembly without having to get their hands or even someone else to help from the other side.”
All rivet tools offer maximum traction force at all times, explains Avery. “The rivet itself decides how much force [it needs], which is determined by a breaker groove on the stem that snaps off at a pre-determined tension. The general assumption with riveting is that you are putting in a rivet into two hard materials, in which case it will break off as required with not too much load. Where you get problems is where other materials, such as plastic, board, or timber products, and that’s when stronger rivets can pull straight through the material.
“That’s where you need a different design of rivet,” he observes. The back end of a peel-type rivet splays into three or four legs that spread out into the rear side of the assembly to hold it secure.
TYPES
Other types of blind rivets include closed-end rivets, which deform within a sealed capsule to help create a watertight joint; multigrip rivets that cover multiple different joint thicknesses by being longer and pulling down to create a bulb on the rear side.
Mechanical fastenings that share some of the features of rivets include lock bolts, which are not blind as they require access from the other side, and rivet nuts, which are installed with a gun but insert full-depth threaded collars. Solid rivets come single-ended, and are slotted through the hole, often while hot, and then held in place while the near end is deformed using a pneumatic hammer.
Every rivet’s performance can be measured in tension – its resistance to pulling in the same axis – and shear, when the two parts are pulled apart laterally, Avery explains. Specifying engineers can meet the required force by choosing the right size and material. With one proviso: “The figures quoted are for one rivet. Joint strength is down to how many you use on a panel. And if you use enough rivets, the failure mode is not with the rivet, but the strength of the panel you’re putting them into. If that is safety-critical, that’s when you do tests.”
Says Keeley: “We’ve done design work with customers working on fascias and panelling, and have had to advise on rivet spacing to make sure that shear and tensile forces are adequate, that there is resistance to environmental wind loads, and room for expansion and contraction.”
The most common rivet material combinations is an aluminium body with a steel mandrel. That is said to offer a good combination of strength, low weight and low cost. Then there are all-steel rivets, which offer more strength. Stainless steel offers not only corrosion resistance, but a little additional strength from work hardening.
Another important factor in rivet performance, contends Keeley, is hole tolerance. Rivetwise advises no more than 0.1mm tolerance in the hole. Any larger and the rivet could slip. However, some rivet types, such as rivets with mandrels that pull through the centre, offer extra tolerance, as they will never pull through, but instead compact extra material to make a larger deformed end.
TOOLING
Pop rivets can be set by hand. Manual pop riveting tools come as either pliers or a multi-leg accordion-style tool. “Hand tools are fine for low-volume work, for hobbyists or if you need one in the repair kit, but it will make your hands and arms ache,” says Avery at Zygology.
Rivet guns have been traditionally pneumatically-powered, such as the Stanley Engineered Fastening ProSet XT4. Inside the hand tool, a spring holds open a set of internal two- or three-part jaws. Pressing the trigger opens a valve allowing in air that forces the nosepiece backwards, which causes the teeth to grip the mandrel and travel with it some 27mm until the mandrel breaks off. The whole cycle happens in a fraction of a second.
Recently, industry has started to move toward battery-powered rivet guns, such as the Stanley Engineered Fastening NBO8PT-18. Says Avery: “There’s been a desire for a few years for battery rivet tools, initially from smaller users, and increasingly larger companies now that battery tools have caught up [mechanically with pneumatic systems]. They like the freedom of not being attached to an air line. Also, a lot of companies are looking to get rid of compressed air systems because of the cost of operating them. It’s not a big step forward in the tool, but it is something the industry is asking for.”
And it can open up new opportunities, too. Avery adds: “Now that battery tools have been developed, we are moving on to a space where the fencing industry can use them instead of hand tools because they can’t get compressed air on site.”
The battery will not last forever, but safeguarding productivity during a shift is as simple as buying a second battery, states Avery. With a recharging time of half an hour, and a duty of perhaps 1,500 placings of a 5mm rivet per charge, it’s extremely unlikely that an operator will drain a battery before the other one is recharged.
According to Keeley, rivet gun customers only tend to jump ship when forced. “On the whole, people are coming away from pneumatic and towards battery power, most commonly because their assets are obsolete or they are having to be repaired.
“Compressed air has a running cost, plus there are compressors to manage. There are health and safety issues from air lines trailing along floors and benches. And there are far less wearing parts in battery-operated tools than pneumatic, which have seals and O-rings to separate air and oil.”
On the other hand, battery-powered tools are 20-30% more expensive than pneumatic tools, are also heavier, perhaps 2kg instead of 1.5kg of pneumatic tools. And when using them, the cut mandrel may fall out of the head of the gun, particularly when used down, while the pneumatic units suck that up into a collection receptacle.
Benefits include requiring no special infrastructure (they are cordless). In addition, Keeley points out that Stanley Engineered Fastening’s Smart Rivet Tool range counts the number of operational cycles, which means that the tool can be pre-set to lock at a certain interval to allow for inspection and maintenance.
In fact the nosepiece can wear, and one sign of that is called ringing on the top of the rivet. That is extra material that has crept up the internal gap in the worn nosepiece. The gun’s jaws also wear out, at perhaps 10,000 placings, states Avery. Maintenance matters. Keeley concludes: “If riveting tools are not working to optimum capacity, they can affect the way that rivets are setting, and cause production disruption as significant as a huge power press going down.”
BOX: Training
At Boeing, new fitting engineers coming through spend days on rivet installation and removal to perfect the technique, states Keeley at Rivetwise, although he admits those are solid, not pop, rivets. He adds: “We offer training on installing equipment and an in-house repair centre. Depending on the customer, we can train the maintenance team to look after the equipment, and how many cycles before the machinery is inspected.”
