At the Gates Corporation Hose Testing Laboratory in Denver, engineers recently evaluated different types of cooling system clamps for their ability to provide higher compression load at different temperatures. Results of that testing are presented in the following report by Gates application engineer Mark Newberry. Cooling system clamps evaluatedThe primary performance characteristic required in fleet and heavy duty cooling system clamps is their ability to provide compression load throughout the operating temperature range. The compressive load must be sufficient enough to be transmitted from the surface of the hose through the wall to create sealing load at the hose/stem interface. Ideally, a clamp should provide a higher compression load at low temperatures so cold leaks are avoided. Under closely controlled laboratory conditions, Gates engineers tested the performance of metal hose clamps and a new polymeric band made from engineering grades of plastics. Metal clamps (see illustrations A & B) were categorized as Constant-Dimension (CD) clamps
Illustration A - Constant Dimension (CD) Clamps
and Spring-Assisted Closure (SAC) clamps (a.k.a., constant tension clamps).
Illustration B - Spring-Assisted Closure (SAC) Clamps
Examples of the CD clamp are the traditional worm-drive clamp, post-type screw clamps and wire-type screw clamps. Examples of the SAC clamp include the spring band, spring-loaded T-bolt and convoluted spring band clamp. Heat shrink technologyThe polymeric bands (PBs) operate through a combination of elastic, thermoplastic memory and matched thermo-linear properties to obtain compression load (see illustration C).
Illustration C - Polymeric Band (PB) Clamps
The heat-sensitive material shrinks to conform to any shape of hose and housing when heat is applied and automatically readjusts every time the engine comes to operating temperature. After the engine is shut down, temperatures decrease. If ambient temperatures drop below freezing, the clamping force increases using the band's increased thermal contraction. Testing involved the installation of clamps and bands over hose on an eight-position load/thermocouple cylinder. The entire assembly was cycled between -20¡F and +200¡F. The relative performance of the best performers in each category were evaluated through the first 10 thermal cycles at the lowest of eight zones around the hose (where a cold leak would first occur). Compression at low temperatureAn examination of load and temperature relationships at the hose stem interface for the three types of clamps revealed that CD clamps reached the no-load cold leak condition at about 45¡F after 10 cycles. Measurements taken throughout the testing shows each cycle's load trending downward, indicating a degradation in load. SAC clamps reached the no-load cold leak condition at just below 0¡F. Although constant tension clamps expand and contract with the hose through temperature cycling, as the spring takes-up the clamp, the spring force is reduced. The PB clamps were by far the best performer with a minimum 20 lb. load, even at -20¡F. A very recent development, these clamps are made of a material with similar thermal contraction properties as the hose wall, enabling the bands to create most of their clamping load at low temperatures. They also have thermal shrink forces built into them which allow the clamp to readjust itself when the engine goes through a heat cycle. Laboratory results involving polymeric band clamps are supported by monitored fleet testing. There has yet to be a cold leak observed in monitored fleet tests using this method. ConclusionOverall, both metal and polymeric clamps were evaluated by Gates engineers for seven different characteristics using a standard A-F grading system (see Table 1). Polymeric bands measurably outperformed metal clamps in maintaining cooling system integrity throughout the operating temperature range and for other evaluated characteristics. Table 1
| Characteristic | CDs | SACs | PBs | | Provide enough load to maintain a seal. | C | B | A | | Not provide too much load so rubber flow is minimized. | C | B | A | | Have over-tightening protection so that hose or clamp damage is avoided. | D | B | A | | Distribute compression load evenly around the hose so no leak-paths are created when cold. | C | C | A | | Be able to readjust itself after every heat cycle so long-term performance is like new. | F | B | A | | Provide higher compression load at low temperature so cold leaks are avoided. | D | B | A | | Be impervious to the degrading effects of its environment. | B | B | A |
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